explain xkcd - User contributions [en]

Talk:2995: University Commas

2024-10-09T12:48:46Z

MelodiousThunk:

As Wikipedia notes, the {{w|Harvard comma}} is actually a thing, and synonymous with the Oxford comma. It's hard to understand whether Randall was just ignoring that.
It's interesting to also look at how the various commas are meaningful. For instance, the Yale comma here appears to be just plain ungrammatical, you'd never put a comma between a verb and a its direct object; similarly the Cambridge comma and Princeton commas are ungrammatical, you'd never put one after the word "and." The Stanford comma is unambiguously normal and it's not clear how you could have such a list without it (absent replacement with a [Stanford?] semicolon). The Columbia comma is being used to separate "mac and cheese" into "mac, and[,] cheese" which changes the semantic meaning (arguably into something meaningless, but maybe we're listing Apple Computers or even Macintosh apple fruit abbreviated). The MIT comma is a cute programming joke for multiline lists. Maybe there are hidden trick meanings (like MIT) I'm missing. [[User:JohnHawkinson|JohnHawkinson]] ([[User talk:JohnHawkinson|talk]]) 23:03, 7 October 2024 (UTC)
:On their own, few of them are intrinsically bad, in the right context.
:*"Please, buy" - valid comma. Prefixed subclause (general plea).
:*"Please buy, apples" - valid comma (more specific plea).
:*"apples, mac" - valid comma (list-type).
:*"mac, and" - valid comma (potentially a conjunctive sub-clause).
:*"mac and, cheese" - valid comma (potentially a post-conjunctive sub-clause).
:*"and cheese, milk" - valid comma (follow-up sub-clause).
:*"cheese, milk, and" - Oxford comma. (Thus invalid, by default. IMO.)
:*"milk and, bread." - ...would be valid, as above, except for the sentence ending.
:*"and bread,." - Ok. Definitely the worst. (Except for the Oxford Comma, which is still worserer!)
:Obviously, combinations of them (or counterpart lack of them, in some cases) can clash badly. Some can work well together, but using ()s, ;s or feetnete* is often better than diving in and out of sub-clauses in the midst of a comma-bound list and potentially making it ambiguous whether you're diving in/out of a clarifying aside or replacing a non-terminating conjunction or perhaps one of the other usages to which a comma might apply.
:* Or just generally rewriting a multi-clausal sentence completely! [[Special:Contributions/172.70.86.22|172.70.86.22]] 23:30, 7 October 2024 (UTC)
::Commas can go in a number of places in lists, and, occasionally, after the word "and". [[User:BunsenH|BunsenH]] ([[User talk:BunsenH|talk]]) 23:34, 7 October 2024 (UTC)
::Potentially, anything is possible... I can see how a sentence like "Please buy apples, mac and cheese, milk, and, bread being out of stock, oats" would work, but I really don't see how the commas after "and" could work ''in this sentence''. [[User:Transgalactic|Transgalactic]] ([[User talk:Transgalactic|talk]]) 08:34, 8 October 2024 (UTC)

If each item in a list shall be followed by a comma then the MIT comma is quite proper. SDT [[Special:Contributions/172.68.245.206|172.68.245.206]] 05:11, 8 October 2024 (UTC)

The UCLA comma may refer to the 8 clap, a chant at UCLA which is begins with a string of 8 claps. {{unsigned ip|172.68.205.178|07:33, 8 October 2024}}

I thought the UCLA & Michigan commas referred to quotes within citations. This isn't uncommon in literary studies, where you quote articles quoting books. Depending on your quotation style, this can result in a long string of 3-4 "commas" (as in: short lines in punctuation marks). If you place the quote between actual commas, make that 4-5. [[User:Transgalactic|Transgalactic]] ([[User talk:Transgalactic|talk]]) 08:34, 8 October 2024 (UTC)

I think the tirade against the Oxford comma in the article is not relevant for understanding the comic. "'To my mother, Ayn Rand and God' does not" is not saying that Ayn Rand is the mother. To express that one should write "To my mother, Ayn Rand, and to God". Thus the ambiguity can be resolved. I believe one of the editors is mixing in their personal taste here. --[[Special:Contributions/172.71.160.71|172.71.160.71]] 09:03, 8 October 2024 (UTC)
:Tirade? Hardly. It explains when it doesn't help (and when it might).
:And I think you misread. "'To my mother, Ayn Rand and God' does not" indeed does not say that Ayn Rand is the mother. In fact it ''explicitly'' says that "'To my mother, Ayn Rand and God'"... erm... does ''not'' say the thing that 'To my mother, Ayn Rand, and God' ''potentially'' does. (See table below.)
:The choice of how to disambiguate "my mother, who is Ayn Rand", as a concept, is another thing and has multiple options. Disambiguating in the direction of a simple list is the contention surrounding the Oxford(/Serial) Comma itself (it is, by definition, being used in the list format), given that some circumstances are most helped by it and others are most helped by its absence. If you're strongly for the OC, you'll hopefully rewrite problematic OCed formulations so that you can use it. If you're strongly against it you should change problamatic non-OCed versions so that you can better go without one. [[Special:Contributions/172.70.85.128|172.70.85.128]] 10:21, 8 October 2024 (UTC)

Inspired (a bit) by the Three Laws permutation table, a set of possible ambiguations from the straight list...
{| class="wikitable"
!A !!B !!C !!"A, B and C" !!"A, B, and C"
|-
|my parents||Ayn Rand||God||"my parents (who are Ayn Rand and God)"||''list only''*
|-
|my parents||God||Ayn Rand||"my parents (who are God and Ayn Rand)"||''list only''*
|-
|Ayn Rand||my parents||God||''list only''* ||''list only''*
|-
|Ayn Rand||God||my parents||''list only''* ||"Ayn Rand (who is God), and my parents"
|-
|God||my parents||Ayn Rand||''list only''* ||''list only''*
|-
|God||Ayn Rand||my parents||''list only''* ||"God (who is Ayn Rand), and my parents"
|}
:-* - Assuming no other "All You Zombies" and/or divine incarnation scenarios.
:...maybe it's too early in the morning, but I'm sure I'm missing other ambiguities I've commented on before. (Without necesarily going into the asterisked territories.) Anyone want to amend this? [[Special:Contributions/172.68.186.105|172.68.186.105]] 09:56, 8 October 2024 (UTC)
::Love it! [[User:Transgalactic|Transgalactic]] ([[User talk:Transgalactic|talk]]) 10:14, 8 October 2024 (UTC)
::There's additional potential ambiguity if you go with the singular "my mother" as opposed to the plural "my parents". "My mother, Ayn Rand, and God" (with the Oxford comma) could be listing 2 separate entities while indicating that my mother is Ayn Rand, or could be listing 3 separate entities. "My mother, Ayn Rand and God" (without the Oxford comma) could be referring to a single entity while indicating that my mother is both Ayn Rand and God, or listing 3 separate entities. (In a phrase like, "My mother, Ayn Rand and God, gave it to me," the comma after God indicates that it's one entity, but you lose that clarity with "It was given to me by my mother, Ayn Rand and God." [[Special:Contributions/172.68.70.66|172.68.70.66]] 14:25, 8 October 2024 (UTC)
:::What if my mother, Ayn Rand, and God are actually the trinity?[[Special:Contributions/172.69.195.87|172.69.195.87]] 08:23, 9 October 2024 (UTC)

I realize that this comic focuses on University commas, however I feel that some mention should be made about the Walken Comma and the Shatner Comma! [[Special:Contributions/172.70.114.103|172.70.114.103]] 10:57, 8 October 2024 (UTC)
:What, do you,    mean by,   that? [[Special:Contributions/172.69.195.106|172.69.195.106]] 13:29, 8 October 2024 (UTC)
::Here's the explanation: [https://www.joeydevilla.com/2015/06/26/a-visual-guide-to-the-different-comma-styles/ Walken and Shatner Commas] [[Special:Contributions/162.158.62.228|162.158.62.228]] 11:43, 9 October 2024 (UTC)

{{w|Mac and cheese}} is probably not well-known outside the US (especially not under that name). --[[Special:Contributions/172.71.160.115|172.71.160.115]] 13:41, 8 October 2024 (UTC)
:As usual, the Brits don't know how to name food. "Macaroni cheese" sounds like the macaroni is made of cheese. But I added an explanation and link to the Wikipedia page. [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 14:30, 8 October 2024 (UTC)
::Don't be silly, that would clearly be named "cheese macaroni". Macaroni cheese is clearly cheese for macaroni, and it's simply polite to serve macaroni to have it with as well. [[Special:Contributions/172.71.151.114|172.71.151.114]] 14:39, 8 October 2024 (UTC)
::: By the logic of your second interpretation, "cheese macaroni" is clearly macaroni for cheese. The lexical existence of this separate form of macaroni begs the question: what kind of macaroni goes best with macaroni cheese? The plain kind or cheese macaroni? The answer is neither! The best kind of macaroni to serve with macaroni cheese is clearly macaroni-cheese macaroni. But then what kind of cheese goes best with that? None other than (macaroni cheese)-macaroni cheese, which in turn is best served with ((macaroni cheese) macaroni)-cheese macaroni. This interleaving of macaroni and cheese never ends, meaning that no matter where you choose to stop, you will always end up with a sub-optimal pairing. So it's best to just not eat any form of cheese with any form of macaroni, to avoid disappointment. As an aside, the logic of your first interpretation implies that "macaroni cheese" is actually cheese that's made of macaroni. [[User:MelodiousThunk|MelodiousThunk]] ([[User talk:MelodiousThunk|talk]]) 12:48, 9 October 2024 (UTC)
:'Mac & cheese' is, sadly, probably more common in the UK now than the proper 'macaroni cheese'.[[Special:Contributions/141.101.99.47|141.101.99.47]] 08:25, 9 October 2024 (UTC)

I'm not opposed to the added red text in the Notation column, but it needs to be explained in the Explanation column. [[Special:Contributions/162.158.90.8|162.158.90.8]] 00:18, 9 October 2024 (UTC)

2599: Spacecraft Debris Odds Ratio

2022-03-30T16:03:52Z

MelodiousThunk: Updated approximation of Q /* Odds & Odds Ratios */

{{comic
| number = 2599
| date = March 28, 2022
| title = Spacecraft Debris Odds Ratio
| image = spacecraft_debris_odds_ratio.png
| titletext = You say this daily walk will reduce my risk of death from cardiovascular disease by 30%, but also increase my risk of death by bear attack by 300%? That's a 280% increased! I'm not a sucker; I'm staying inside.
}}

==Explanation==
{{incomplete|Created by an EVENS RATIO - Do NOT delete this tag too soon.}}
This comic is a misunderstanding of statistics very similar to that of [[1252: Increased Risk]]. It explains that going outside for more than 5 hours per day significantly increases your risk of head injury from falling spacecraft, and advises to limit outside activity to avoid this risk.

However, since the odds of being hit in the head by (any part of) a falling spacecraft are astronomically low to begin with [https://www.livescience.com/33511-falling-nasa-satellite-uars-risk.html], quadrupling it or more still results in a negligible probability. The horizontal error bars for times greater than 4 hours are marked with asterisks to indicate they are significantly different from the reference value at 0 hours, as indeed those error bars don't overlap the vertical line for the 0-hours reference value.

{{w|Error bar}}s are graphical representations of the variability of data and used on graphs to indicate the error or uncertainty in a reported measurement.

Presenting the data by hour brackets hides the data distribution inside each bracket. If the data were presented hour by hour, and not by groups of hours, they may show a different threshold of increased risk or no threshold (odds ratio could be linear).

The graph and error bars are based on a {{w|Monte Carlo Method|Monte Carlo simulation}}, a type of computational algorithm that uses repeated random sampling to obtain the likelihood of a range of results of occurring; see, for instance, this article about [https://www.ibm.com/cloud/learn/monte-carlo-simulation Monte Carlo simulations]. Additionally, this may indicate that the entire study was conducted via a monte carlo simulation and that no real data was collected adding to the absurdity of the claim that more time spent outside could lead to an increased risk of head injuries due to falling space craft. Indeed, it is so rare for humans to be struck by spacecraft debris that a simulation is probably the only way to study the risk; an absurdly large sample size, involving tens of millions of participants over several decades, would be necessary to obtain significant experimental data.

The specific reference to falling spacecraft is likely inspired by events happening around the time of this comics release (March 2022). Around a month before this was posted, the head of the Russian space agency, {{w|Roscosmos}}, warned that sanctions against Russia (mostly those over the {{w|2022 Russian invasion of Ukraine}}) could result in the {{w|International Space Station}} crashing. Since the Russian section of the space station is the one that provides propulsion (although it is built to rely on the power generated by the other sections), this was taken seriously and as of when this was posted, {{w|NASA}} was trying to come up with alternative stabilization strategies in case the situation worsened. There was also a recent [https://www-uol-com-br.translate.goog/tilt/noticias/redacao/2022/03/17/parte-do-foguete-spacex-e-encontrada-por-morador-do-pr.htm?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=pt-BR&_x_tr_pto=wapp report] of some 600 kg space rocket debris found in Brazil.

The title text makes a similar joke. While the increase in chances of death by a bear attack are greater when going outside than the decrease in chances of death by cardiovascular disease, by getting out to exercise, it is incorrect to combine them in this way, since cardiovascular disease has a much higher starting chance of death, and reducing it by 30% has a much more significant effect on overall life expectancy than quadrupling the very very small chance of death by bear attack.

The "280% increase" of the title text is also an error, though perhaps not for reasons that are obvious at first (for instance, the correct calculation is not "300% − 30% = 270%"). To "increase by 300%" means multiplying the probability by (1 + 3.0) = 4.0, while to "decrease by 30%" means multiplying by (1 − 0.3) = 0.7. Combining these means multiplying by both, for an overall change of 4.0 × 0.7 = 2.8, or 280%. However, this result means the risk has increased ''to'' 280% of its old value, not ''by'' 280%. And in any case, it is ''still'' not valid to simply combine two changes in wildly different risks like this.

===Odds & Odds Ratios===

The odds of an event is the probability that it happens divided by the probability that it doesn't happen. People often express odds as a ratio (e.g. the odds of rolling a 6 on a 6-sided dice might be expressed as 0.16777... : 0.83333..., or equivalently as 1:5), but it is important to note that such ratios are not ''odds ratios'' (it would be fitting to call this a "probability ratio", but this terminology is not standard).

An odds ratio is the odds of event O happening, given that some other event E has occurred, divided by the odds of O given that E has not occurred. O is sometimes called an "outcome" and E is sometimes called an "exposure", because people are often interested in comparing things like the odds of getting lung cancer (O) given that you smoke (E) to the odds of getting lung cancer given that you don't smoke, as a way of measuring the extent to which exposure to E influences outcome O. In the case of the comic, the outcome variable O is the event of getting a head injury from falling spacecraft debris, and the exposure variable E is the event of spending H hours per day outside, for various values of H. The comic appears to be saying that for each value of H, there are two options for E: either you spend H hours per day outside or you never go outside.

So for small values of H (e.g. 1 hour per day), the comic is saying that the event of being hit by spacecraft debris is more or less independent of the event of spending H hours per day outside, which is to say that the odds of being hit is more or less the same regardless of the choice you make between spending H hours per day outside and never going outside. Hence the dot on the 1-hour bar is close to 1, because the two odds are more or less equal (the dot appears to represent an average estimate of the odds ratio).

Note that when calculating the odds ratios for this comic, the odds in the denominators are always the same, as they are the odds of being hit given that you never go outside, which does not depend on H. So when the comic says that the odds ratio is above 3 for H={11+ hours per day}, it is effectively saying that the odds of being hit when you spend this much time outside is a bit more than 3 times the odds of being hit when you spend 1 hour per day outside.

Suppose the probability of being hit is: P when you spend 1 hour per day outside, and Q when you spend 11+ hours per day outside. The odds of being hit under these two exposures are P/(1-P) and Q/(1-Q) respectively, and because the odds ratios have equal denominators, the comic is saying that Q/(1-Q) = kP/(1-P), where k is a bit more than 3. If we rearrange this to get an expression for Q, we get:

<pre>
Q/(1-Q) = kP/(1-P)
<=> Q(1-P) = kP(1-Q)
<=> Q-QP = kP-kPQ
<=> Q+kPQ-QP = kP
<=> Q(1+kP-P) = kP
<=> Q = kP/(1+kP-P)
<=> Q = P/(P+(1-P)/k) {by dividing the numerator and denominator by k}
</pre>

As P is negligibly small, 1-P is very close to 1, and P+(1-P)/k is very close to 1/k. Thus Q is very close to kP (i.e. a bit more than 3 times P), meaning that the probability of being hit when you spend 11+ hours per day outside is still negligibly small. Thus, the comic's suggestion that we spend 4 hours or less outside based on the estimated odds ratios is extremely misguided.

==Transcript==
:[A chart is shown. Above the chart there is a heading, with a subheading below it:]
:Odds ratio for head injuries from falling spacecraft debris
:(Monte Carlo Simulation)

:[The chart is rectangular with the X-axis labels above the chart with numbers from 1 to 5. These are places over vertical lines. The first at 1 is black, the other four are light gray. There are three smaller light gray ticks between each set of lines, and one on either side of the first and last. The distance between lines gets smaller and smaller towards the right, probably logarithmic.]
:X-axis: 1 2 3 4 5

:[The Y-axis is not scaled; there are no ticks or lines. Instead it just gives five labels from top to bottom. Above those labels there is an arrow pointing to the top one with a label above explaning the axis.]
:Hours spent outdoors per day
:Y-axis:
::0 (ref)
::1
::2-4
::5-10
::11+

:[Aligned with each of these five divisions of the Y-axis there is a dot. The top one is placed on the solid line under 1 as a reference point. The other four dots all have long error bars, with the dots at the center of these. The second dot is a bit to the left of the solid line, with the error bar going almost to the left edge of the graph and halfway to the first light gray line to the right. The third dot is located halfway between the solid and the first light gray line with the error bar just crossing the solid line, and almost reaching the gray line. The fourth dot is about a third way between the first and second of the gray lines, with the error bar crossing both these lines. The fifth and last dot is just past the second gray line, with the error bar crossing both that, going more than half toward the first gray line, and also just past the third gray line. On the same height as the two bottom dots, there are asterisks just right of the edge of the graph.]
:<nowiki>*</nowiki>
:<nowiki>*</nowiki>

:[Below the panel there is a caption:]
:Our new study suggests that spending more than 5 hours outside significantly increases your risk of head injury from spacecraft debris, so try to limit outdoor activities to 4 hours or less.

==Trivia==
*In the [https://www.explainxkcd.com/wiki/images/archive/d/d5/20220329223238%21spacecraft_debris_odds_ratio.png original version] of the comic the Y-axis label referred to "hours spent outdoors". So more than four hours spent outdoors in one's lifetime would be a problem.
**But later the comic was edited to specify "hours spent outdoors per day", which makes more sense.
*When the new version was uploaded, Randall again made the error of making the two versions of the comic image the same size, as he did earlier in [[2576: Control Group]], see that comic's [[2576: Control Group#Trivia|trivia]]
**This resulted in the problem that the comic broke the boundaries on the xkcd website.
**This was later fixed. But even at that time, the two images was the same size.
**Here an example of how it looked when the error was present:
[[File:2599- Spacecraft Debris Odds Ratio Image scaling off.png|500px]]

{{comic discussion}}

[[Category:Charts]]
[[Category:Statistics]]
[[Category:Space]]
[[Category:Animals]]

Talk:2599: Spacecraft Debris Odds Ratio

2022-03-30T16:00:08Z

MelodiousThunk: Notification about new "Odds & Odds Ratios" section.

correct me if i'm wrong, but i believe 300 - 30 is 270, not 280? [[Special:Contributions/172.68.50.85|172.68.50.85]] 22:50, 28 March 2022 (UTC)
:something something percentage points maybe? idk [[Special:Contributions/172.70.134.91|172.70.134.91]] 22:56, 28 March 2022 (UTC)Bumpf
:Most likely there is an unstated chance of death by not going outside... presumably ~10% but there's no way to know the breakdown (could be nearly all cardio, could be nearly all ursine if they live in a cave next bears) [[Special:Contributions/172.69.70.127|172.69.70.127]] 23:02, 28 March 2022 (UTC)
::300% increase is multiplying by (1+3), 30% decrease is multiplying by (1-0.3) , %increases are multiplicative so the increase is by a factor of 4*0.7=2.8, which is 280% of the original value (or a 180% increase). {{unsigned ip|162.158.146.69}}
:::Yeah, barring a total mistake, that must be where the number came from, but it seems odd by the inconsistent way it is expressed, as it assumes the 300% increase for the bear attack is added to the initial value for a final amount of 400%, along with a similar treatment for the 30% decrease, but the 280% is simply the final value skipping past that step to the conclusion afterwards that is not even shown for the previous numbers. But with the improper grammar, if it's not an actual typo, it may be trying to show the speaker acting dumb or irrational, as it doesn't make sense to end with "increased" instead of "increase" without changing part of the words before that number. Someone thinking that poorly though likely wouldn't be able to multiply things properly to produce that 280% number though.--[[Special:Contributions/172.70.130.153|172.70.130.153]] 01:13, 29 March 2022 (UTC)
::::Someone who do understand this method of getting to 280% should add that to the explanation. I'm not quite sure what is meant here above, so an even better explanation would be preferable. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 08:28, 29 March 2022 (UTC)
:::Joke proof: ''Assume'' that every year 400 people are killed by bears in the world, of which 100 are killed inside and 300 are killed outside. Then, indeed, by going outside, the probability that you will be killed by bears increases from 100 to 300: that is 300%. On the other hand, we know that walking outside every day will reduce your risk of death from cardiovascular disease by 30%. Therefore, by walking outside properly, 30% of the above-mentioned 400 people, i.e. 120 people, could in theory avoid death from the said disease, ''if'' not attacked by bears. This implies that, even if everyone in the world walked outside every day, only 120 out of the 400 bear attack victims would be potentially saved, while 280 would die anyway. Since by hypothesis only 100 are killed inside by bear attacks, going outside will clearly increase the probability of deadly bear attacks, from 100 to 280: that is 280%. —[[User:Yosei|Yosei]] ([[User talk:Yosei|talk]]) 09:52, 29 March 2022 (UTC)
:::As said above, 300% increase and 30% decrease gives a factor ×2.8 which is a +180% increase (not 280%) [[Special:Contributions/162.158.50.176|162.158.50.176]] 10:38, 29 March 2022 (UTC)
::::It's a joke :) Since the title text is obviously a joke, maybe we shouldn't over-analyze it, except we can enjoy ourselves by “analyzing” it half-jokingly. Seriously, though, there is also some ambiguity in a natural language itself: e.g. by “one-and-a-half times larger than”, one may mean “one-and-a-half times as large as” (150%), or one may mean “150% larger than” (250%). When spoken informally, this kind of ambiguity is not uncommon. Another example would be “five hundred one thousandths” which may mean 501/1000 or 500/1000. Take it easy & take care :) — [[User:Yosei|Yosei]] ([[User talk:Yosei|talk]]) 11:38, 29 March 2022 (UTC)
:::This is what I love about XKCD, the jokes come with proofs. Does it depend on what order you apply them in? If you decrease the risk by 30%, you have 70%, then increase it by 300%, you get... 210%? Or 270%? Percentage points vs. percent again isn't it. Why is life so complicated? --[[User:192·168·0·1|192·168·0·1]] ([[User talk:192·168·0·1|talk]]) 12:46, 29 March 2022 (UTC)
:It doesn't really matter because the whole thing is complete nonsense. You can't combine the risks unless you know how big they are relative to each other. Let's say 1,000 people stay inside. 2 are killed by a bear and 10 die of cardiovascular disease - 12 people in total. With the given percentage changes, of 1,000 people who go outside, 8 get killed by bears (300% increase) and 7 die of heart disease (30% decrease), a total of 15. It's more dangerous to go outside than stay in. However, if 250 of the people who stay inside die of heart disease, then we have 252 deaths in total for staying in and only 175+8=183 for going out [[User:Jeremyp|Jeremyp]] ([[User talk:Jeremyp|talk]]) 15:33, 29 March 2022 (UTC).

"That's a 280% increased" has a typo/grammaro. The last word should be "increase". [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 23:04, 28 March 2022 (UTC)
:I think the actual typo is the "a" so should be "That's 280% increased" {{unsigned ip|162.158.146.69}}
::In standard American grammar it is much more likely that he meant "That's a 280% increase" than "That's 280% increased." You might say the odds ratio that he meant the former over the latter is 3+.[[Special:Contributions/162.158.166.87|162.158.166.87]] 15:46, 29 March 2022 (UTC)

Also what's an odds ratio?? ~~Bumpf {{unsigned ip|172.70.38.41}}
:I assume something like "million to one". But the units of the horizontal axis clearly don't correspond to that. I don't know what those units are, they're not a percentage, either. [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 00:40, 29 March 2022 (UTC)
:: if you say "this is 4 times as likely" then "4" is the "odds ratio", this is the type of number appearing on the horizontal axis {{unsigned ip|162.158.146.69}}
:An odds-ratio is a way of reporting the results for predictions of binary outcomes. It's a transformation of the (not easily interpretable) regression coefficient. For example, if the OR for "males" (vs females) is "0.70", they're 70% as likely to have the outcome as females; if it's "1.32", then males are 1.32x as likely (equivalently: 32% more likely) to have that outcome as females. [[Special:Contributions/108.162.249.75|108.162.249.75]] Gye Greene

Did something happen to the size of the image after the initial posting? [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 00:40, 29 March 2022 (UTC)

What's with the asterisks on the right side? [[User:Jordan Brown|Jordan Brown]] ([[User talk:Jordan Brown|talk]]) 00:50, 29 March 2022 (UTC)
:I think the asterisks denote that the value at this range is "significant" because its error bars do not overlap with the baseline. If you stay outdoors 5 hours or more in a day, there is a nonzero chance that you will be hit by flying space debris. [[User:Laura|Laura]] ([[User talk:Laura|talk]]) 08:15, 29 March 2022 (UTC)

There should probably be an explanation of what "Monte Carlo Simulation" means, as many people who would actually want an explanation of this strip would likely be unfamiliar with that term.--[[Special:Contributions/172.70.131.122|172.70.131.122]] 01:02, 29 March 2022 (UTC)
:Yes, exactly! I got as far as finding {{w|Monte Carlo method}} via a redirect but have no idea how the bars are supposed to work, what the reference point is supposed to mean, or why the columns get skinnier toward the right. Not dumb, but next to no statistics education. [[User:Yngvadottir|Yngvadottir]] ([[User talk:Yngvadottir|talk]]) 07:51, 29 March 2022 (UTC)

::Yes, I added some links to try to make the graph a little more explore-friendly for folks willing to click and read what's beyond, but I don't have the smarts to really explain it. [[User:Laura|Laura]] ([[User talk:Laura|talk]]) 08:00, 29 March 2022 (UTC)

Why is the x-axis of the chart in logarithmic spacing? Any particular reason for this, or is it part of the joke? [[User:Captain Nemo|Captain Nemo]] ([[User talk:Captain Nemo|talk]]) 09:29, 29 March 2022 (UTC)

I wonder if it's deliberate that there's actually less risk if you go outside 1 hour per day. --[[User:192·168·0·1|192·168·0·1]] ([[User talk:192·168·0·1|talk]]) 12:46, 29 March 2022 (UTC)

Is this covid commentary? Like how everyone got freaked about the odds for covid to the point where they stopped exercising and shutting everyone inside and degrading their mental health? [[Special:Contributions/172.70.131.122|172.70.131.122]] 18:26, 29 March 2022 (UTC)

;Odds ratio confusion?
I am very confused by the X axis of this comic, I feel like I must be misunderstanding how this works, but I thought I understood how odds ratios worked. Maybe not.
The graph "reads" that "In the reference situation, with zero hours spent outside, the odds ratio for head injuries from falling spacecraft debris is 1.0 ± 0." A 1.0 odds ratio means 1.0:1.0, or that either possibility is 50% likely. That is, there's an even chance your head will be injured by spacecraft debris or that it will not, ''if you stay indoors.'' That does not seem like it could be right, so can someone point me to my error? Thanks! [[User:JohnHawkinson|JohnHawkinson]] ([[User talk:JohnHawkinson|talk]]) 09:34, 29 March 2022 (UTC)
:As best I can tell, this is taking odds as a ratio between ''any'' two events. Rather than the usual "success : failure" (or "happens : doesn't happen"), it's "this scenario happens : control scenario happens". By definition, the control scenario is set at 1.0, and something at a ratio of (say) 2.0 is twice as likely to happen. -- [[User:Peregrine|Peregrine]] ([[User talk:Peregrine|talk]]) 10:50, 29 March 2022 (UTC)
::I definitely think we need to put something explaining what an odds ratio is. But since I feel the need to have it explained, I'm not going to be the one to explain it. --[[User:192·168·0·1|192·168·0·1]] ([[User talk:192·168·0·1|talk]]) 12:46, 29 March 2022 (UTC)
::: I've added an "Odds & Odds Ratios" section to the comic. Does it clear things up? [[User:MelodiousThunk|MelodiousThunk]] ([[User talk:MelodiousThunk|talk]]) 16:00, 30 March 2022 (UTC)

;Per day
Looks like the comic has been updated to clarify that the number of hours is per day. I'll leave it to someone more experienced with this website to update it, but in any case it makes the note "It is very difficult to avoid being outside for more than four hours in a total lifetime" moot. [[Special:Contributions/172.70.114.147|172.70.114.147]] 12:31, 29 March 2022 (UTC)
:I uploaded the new version that includes "per day" in the y-axis label. But the image size also changed, now the image is the normal _2x size. I'm hoping that will get fixed eventually, like it did for [[2576: Control Group]]. [[User:Orion205|Orion205]] ([[User talk:Orion205|talk]]) 22:42, 29 March 2022 (UTC)
::I have uploaded a version of normal size, that I have scaled myself. And moved the mention of this to a new trivia. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 06:32, 30 March 2022 (UTC)

;Monte Carlo Tree Searches
MCTSs are one of those things that don't seem like they should work but they do [[User:Beanie|Beanie]] [[User talk:Beanie|talk]] 20:55, 29 March 2022 (UTC)
:I just did my own Monte Carlo Tree Search and... [https://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Monaco_Monte_Carlo_1.jpg/800px-Monaco_Monte_Carlo_1.jpg there's definitely at least one, jutting up into the bottom/right of that overview]. :-p [[Special:Contributions/172.70.91.36|172.70.91.36]] 22:37, 29 March 2022 (UTC)

;Image scaling off
Does anyone else experience a problem with the scaling of the comic image? It is not fitting to the frame, but displays on full size on the web page. It only happens for this comic, not other ones, and i see it both on the main page as the xkcd/2599 page. Some mistake for sure, but I have not seen this before. Screenshot proof: [https://i.imgur.com/sbXbCov.png imgur link] [[User:Flekkie|Flekkie]] ([[User talk:Flekkie|talk]]) 22:32, 29 March 2022 (UTC)
:This happened back in [[2576: Control Group]]. It was fixed after about a week. [[User:Orion205|Orion205]] ([[User talk:Orion205|talk]]) 22:42, 29 March 2022 (UTC)
::I have mentioned this in a new trivia section and added the picture as example. I will add ref to [[2576]] also now. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 06:32, 30 March 2022 (UTC)

The error with the really big image is still present for me. [[Special:Contributions/172.69.90.77|172.69.90.77]] 14:24, 30 March 2022 (UTC)

2599: Spacecraft Debris Odds Ratio

2022-03-30T15:57:54Z

MelodiousThunk: Added "Odds & Odds Ratios"

{{comic
| number = 2599
| date = March 28, 2022
| title = Spacecraft Debris Odds Ratio
| image = spacecraft_debris_odds_ratio.png
| titletext = You say this daily walk will reduce my risk of death from cardiovascular disease by 30%, but also increase my risk of death by bear attack by 300%? That's a 280% increased! I'm not a sucker; I'm staying inside.
}}

==Explanation==
{{incomplete|Created by an EVENS RATIO - Do NOT delete this tag too soon.}}
This comic is a misunderstanding of statistics very similar to that of [[1252: Increased Risk]]. It explains that going outside for more than 5 hours per day significantly increases your risk of head injury from falling spacecraft, and advises to limit outside activity to avoid this risk.

However, since the odds of being hit in the head by (any part of) a falling spacecraft are astronomically low to begin with [https://www.livescience.com/33511-falling-nasa-satellite-uars-risk.html], quadrupling it or more still results in a negligible probability. The horizontal error bars for times greater than 4 hours are marked with asterisks to indicate they are significantly different from the reference value at 0 hours, as indeed those error bars don't overlap the vertical line for the 0-hours reference value.

{{w|Error bar}}s are graphical representations of the variability of data and used on graphs to indicate the error or uncertainty in a reported measurement.

Presenting the data by hour brackets hides the data distribution inside each bracket. If the data were presented hour by hour, and not by groups of hours, they may show a different threshold of increased risk or no threshold (odds ratio could be linear).

The graph and error bars are based on a {{w|Monte Carlo Method|Monte Carlo simulation}}, a type of computational algorithm that uses repeated random sampling to obtain the likelihood of a range of results of occurring; see, for instance, this article about [https://www.ibm.com/cloud/learn/monte-carlo-simulation Monte Carlo simulations]. Additionally, this may indicate that the entire study was conducted via a monte carlo simulation and that no real data was collected adding to the absurdity of the claim that more time spent outside could lead to an increased risk of head injuries due to falling space craft. Indeed, it is so rare for humans to be struck by spacecraft debris that a simulation is probably the only way to study the risk; an absurdly large sample size, involving tens of millions of participants over several decades, would be necessary to obtain significant experimental data.

The specific reference to falling spacecraft is likely inspired by events happening around the time of this comics release (March 2022). Around a month before this was posted, the head of the Russian space agency, {{w|Roscosmos}}, warned that sanctions against Russia (mostly those over the {{w|2022 Russian invasion of Ukraine}}) could result in the {{w|International Space Station}} crashing. Since the Russian section of the space station is the one that provides propulsion (although it is built to rely on the power generated by the other sections), this was taken seriously and as of when this was posted, {{w|NASA}} was trying to come up with alternative stabilization strategies in case the situation worsened. There was also a recent [https://www-uol-com-br.translate.goog/tilt/noticias/redacao/2022/03/17/parte-do-foguete-spacex-e-encontrada-por-morador-do-pr.htm?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=pt-BR&_x_tr_pto=wapp report] of some 600 kg space rocket debris found in Brazil.

The title text makes a similar joke. While the increase in chances of death by a bear attack are greater when going outside than the decrease in chances of death by cardiovascular disease, by getting out to exercise, it is incorrect to combine them in this way, since cardiovascular disease has a much higher starting chance of death, and reducing it by 30% has a much more significant effect on overall life expectancy than quadrupling the very very small chance of death by bear attack.

The "280% increase" of the title text is also an error, though perhaps not for reasons that are obvious at first (for instance, the correct calculation is not "300% − 30% = 270%"). To "increase by 300%" means multiplying the probability by (1 + 3.0) = 4.0, while to "decrease by 30%" means multiplying by (1 − 0.3) = 0.7. Combining these means multiplying by both, for an overall change of 4.0 × 0.7 = 2.8, or 280%. However, this result means the risk has increased ''to'' 280% of its old value, not ''by'' 280%. And in any case, it is ''still'' not valid to simply combine two changes in wildly different risks like this.

===Odds & Odds Ratios===

The odds of an event is the probability that it happens divided by the probability that it doesn't happen. People often express odds as a ratio (e.g. the odds of rolling a 6 on a 6-sided dice might be expressed as 0.16777... : 0.83333..., or equivalently as 1:5), but it is important to note that such ratios are not ''odds ratios'' (it would be fitting to call this a "probability ratio", but this terminology is not standard).

An odds ratio is the odds of event O happening, given that some other event E has occurred, divided by the odds of O given that E has not occurred. O is sometimes called an "outcome" and E is sometimes called an "exposure", because people are often interested in comparing things like the odds of getting lung cancer (O) given that you smoke (E) to the odds of getting lung cancer given that you don't smoke, as a way of measuring the extent to which exposure to E influences outcome O. In the case of the comic, the outcome variable O is the event of getting a head injury from falling spacecraft debris, and the exposure variable E is the event of spending H hours per day outside, for various values of H. The comic appears to be saying that for each value of H, there are two options for E: either you spend H hours per day outside or you never go outside.

So for small values of H (e.g. 1 hour per day), the comic is saying that the event of being hit by spacecraft debris is more or less independent of the event of spending H hours per day outside, which is to say that the odds of being hit is more or less the same regardless of the choice you make between spending H hours per day outside and never going outside. Hence the dot on the 1-hour bar is close to 1, because the two odds are more or less equal (the dot appears to represent an average estimate of the odds ratio).

Note that when calculating the odds ratios for this comic, the odds in the denominators are always the same, as they are the odds of being hit given that you never go outside, which does not depend on H. So when the comic says that the odds ratio is above 3 for H={11+ hours per day}, it is effectively saying that the odds of being hit when you spend this much time outside is a bit more than 3 times the odds of being hit when you spend 1 hour per day outside.

Suppose the probability of being hit is: P when you spend 1 hour per day outside, and Q when you spend 11+ hours per day outside. The odds of being hit under these two exposures are P/(1-P) and Q/(1-Q) respectively, and because the odds ratios have equal denominators, the comic is saying that Q/(1-Q) = kP/(1-P), where k is a bit more than 3. If we rearrange this to get an expression for Q, we get:

<pre>
Q/(1-Q) = kP/(1-P)
<=> Q(1-P) = kP(1-Q)
<=> Q-QP = kP-kPQ
<=> Q+kPQ-QP = kP
<=> Q(1+kP-P) = kP
<=> Q = kP/(1+kP-P)
<=> Q = P/(P+(1-P)/k) {by dividing the numerator and denominator by k}
</pre>

As P is negligibly small, 1-P is very close to 1, and P+(1-P)/k is close to 1/3. Thus Q is approximately 3P, meaning that the probability of being hit when you spend 11+ hours per day outside is still negligibly small. Thus, the comic's suggestion that we spend 4 hours or less outside based on the estimated odds ratios is extremely misguided.

==Transcript==
:[A chart is shown. Above the chart there is a heading, with a subheading below it:]
:Odds ratio for head injuries from falling spacecraft debris
:(Monte Carlo Simulation)

:[The chart is rectangular with the X-axis labels above the chart with numbers from 1 to 5. These are places over vertical lines. The first at 1 is black, the other four are light gray. There are three smaller light gray ticks between each set of lines, and one on either side of the first and last. The distance between lines gets smaller and smaller towards the right, probably logarithmic.]
:X-axis: 1 2 3 4 5

:[The Y-axis is not scaled; there are no ticks or lines. Instead it just gives five labels from top to bottom. Above those labels there is an arrow pointing to the top one with a label above explaning the axis.]
:Hours spent outdoors per day
:Y-axis:
::0 (ref)
::1
::2-4
::5-10
::11+

:[Aligned with each of these five divisions of the Y-axis there is a dot. The top one is placed on the solid line under 1 as a reference point. The other four dots all have long error bars, with the dots at the center of these. The second dot is a bit to the left of the solid line, with the error bar going almost to the left edge of the graph and halfway to the first light gray line to the right. The third dot is located halfway between the solid and the first light gray line with the error bar just crossing the solid line, and almost reaching the gray line. The fourth dot is about a third way between the first and second of the gray lines, with the error bar crossing both these lines. The fifth and last dot is just past the second gray line, with the error bar crossing both that, going more than half toward the first gray line, and also just past the third gray line. On the same height as the two bottom dots, there are asterisks just right of the edge of the graph.]
:<nowiki>*</nowiki>
:<nowiki>*</nowiki>

:[Below the panel there is a caption:]
:Our new study suggests that spending more than 5 hours outside significantly increases your risk of head injury from spacecraft debris, so try to limit outdoor activities to 4 hours or less.

==Trivia==
*In the [https://www.explainxkcd.com/wiki/images/archive/d/d5/20220329223238%21spacecraft_debris_odds_ratio.png original version] of the comic the Y-axis label referred to "hours spent outdoors". So more than four hours spent outdoors in one's lifetime would be a problem.
**But later the comic was edited to specify "hours spent outdoors per day", which makes more sense.
*When the new version was uploaded, Randall again made the error of making the two versions of the comic image the same size, as he did earlier in [[2576: Control Group]], see that comic's [[2576: Control Group#Trivia|trivia]]
**This resulted in the problem that the comic broke the boundaries on the xkcd website.
**This was later fixed. But even at that time, the two images was the same size.
**Here an example of how it looked when the error was present:
[[File:2599- Spacecraft Debris Odds Ratio Image scaling off.png|500px]]

{{comic discussion}}

[[Category:Charts]]
[[Category:Statistics]]
[[Category:Space]]
[[Category:Animals]]