explain xkcd - User contributions [en]

2545: Bayes' Theorem

2021-11-22T23:01:29Z

108.162.219.27: /* Explanation */ took a stab at explaining the title text more in depth

{{comic
| number = 2545
| date = November 22, 2021
| title = Bayes' Theorem
| image = bayes_theorem.png
| titletext = <nowiki>P((B|A)|(A|B)) represents the probability that you'll mix up the order of the terms when using Bayesian notation.</nowiki>
}}

==Explanation==
{{incomplete|Created by a FALSE POSITIVE - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

{{w|Bayes' theorem}} describes the probability of an event, based on prior knowledge of conditions that might be related to the event.

In this comic, the off-panel student knows that they are studying Bayes' theorem, so they use that prior knowledge to guess that the test result is a false positive. The punch line is the caption - if you know Bayes' theorem well enough, you don't need to actually calculate the probabilities.

The title text refers to the mathematical definition of Bayes' theorem: P(A | B) = P(B|A) * P(A) / P(B). P(A|B) represents the probability of some event A occurring, given that B has occurred. This is often referred to as "the probability of A given B". It can be hard to remember if P(A|B) means probability of A given B, or if it's B given A, and Randall's joke is based on this difficulty. Specifically, P((A|B)|(B|A)) would be the probability of (A|B) given (B|A), which makes it the probability that you got the order of the notation mixed up.

==Trivia==
When this comic came out, the title text was only "P((B", but this was quickly corrected.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:[Miss Lenhart using a pointer and pointing to a white-board with statistical formulae]
:Miss Lenhart: Given these prevalences, is it likely that the test result is a false positive?
:(off-panel voice): Well, this chapter is on Bayes' Theorem, so yes.

:[Caption below the panel]:
:Sometimes, if you understand Bayes' Theorem well enough, you don't need it.

{{comic discussion}}

[[Category:Comics featuring Miss Lenhart]]
[[Category:Statistics]]

Talk:1559: Driving

2015-08-06T12:34:57Z

108.162.219.27:

They're probably in California, seeing as that's the only place self-driving cars are actually on the road. [[User:Wmss|Wmss]] ([[User talk:Wmss|talk]]) 09:46, 3 August 2015 (UTC)

If there are self-driving cars, what about self-filling cars? So these cars are able to make long distances without the driver's interaction. Maybe he is sleeping. [[User:GeorgDerReisende|GeorgDerReisende]] ([[User talk:GeorgDerReisende|talk]]) 10:47, 3 August 2015 (UTC)
: Yeah, for some reason that does not exist yet -- the self-driving car on the other hand DOES exit and I can see them driving down my street every day [[User:Spongebog|Spongebog]] ([[User talk:Spongebog|talk]]) 01:07, 4 August 2015 (UTC)

::People have no problem entrust the self-driving cars with their lives, but did you saw how much the petrol costs? Too risky. -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 10:26, 4 August 2015 (UTC)

Should've used hitchBOT instead of a rock. [[Special:Contributions/108.162.216.82|108.162.216.82]] 12:46, 3 August 2015 (UTC)

Could this have been a semi-tribute? Showing how some malicious people will abuse technology that is programmed to be too trusting? [[Special:Contributions/108.162.216.138|108.162.216.138]] 20:56, 3 August 2015 (UTC)
: Could you explain with an example [[User:Spongebog|Spongebog]] ([[User talk:Spongebog|talk]]) 01:07, 4 August 2015 (UTC)

I disagree with the part of the explanation that claims that the comic must take place in the continental US. The title text doesn't specify "exactly two" border crossings; it merely implies that there is more than one. That could be anywhere in mainland (or attached-to-the-mainland-by-bridge) North America, north of the Darien Gap, except for most of Canada (from most of the population centers of Ontario, Google Maps wants to route through Michigan, for a total of three border crossings). [[Special:Contributions/108.162.221.149|108.162.221.149]] 14:18, 3 August 2015 (UTC)
:Correct. It "implies" more than one, but doesn't require it. Only that there be at least one. So they could be in Canada. Essentially anywhere on the continent (or an island connected by bridge to the mainland (e.g., Florida Keys)) north of the Darien Gap but outside Alaska. - Equinox [[Special:Contributions/108.162.238.172|108.162.238.172]] 23:15, 3 August 2015 (UTC)
::I disagree. Randall makes comic in the mainland US and if there is no indication we are outside it is safe to assume they are in the mainland US. And the title text clearly indicated more than one border crossing. You are making it way to complicated. I have corrected accordingly.--[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 12:02, 4 August 2015 (UTC)

It is likely that the car's owner can locate it via the Internet, via an app and location logic provided by the car's manufacturer. E.g., OnStar. {{unsigned ip|108.162.216.157}}

"At the time of the release of this comic there were no places where these cars could be used privately." That is not true, as there are no restrictions on vehicle use on private property. --[[Special:Contributions/108.162.212.7|108.162.212.7]] 16:35, 3 August 2015 (UTC)
: You are correct, and I have fixed that -- the true statement is that they are not for sale to private individuals [[User:Spongebog|Spongebog]] ([[User talk:Spongebog|talk]]) 01:03, 4 August 2015 (UTC)

The only US state that both borders Canada and permits self-driving cards is Michigan. Assuming that the logic in self-driving cars prevents them from driving on streets where they are not legal, the conversation would have to take place in that state (but then again, wouldn't the car know that it is not allowed to drive in Alaska?) [[Special:Contributions/162.158.92.64|162.158.92.64]] 19:38, 3 August 2015 (UTC)
:Maybe it's exactly a matter of what knowledge the car has. It ''could'' be linked up to something proprietry, or possibly a Cortana information-engine, to control a virtual "GPS fence", based on current legal and possibly licence-based limits. I'll bet it can be reprogrammed to ignore/extend such limits, though. (Which is why I'm dubious about the idea of 'hard limiting' flying drones from entering restricted airspace. A little hardware/software/firmware hacking should be simple enough for anyone who needs to get around such limits.)
:I can also imagine the following conversation: "How far does your car's self-driving system let you go on automatic?" "I'll ask her..."(/Alaska...) [[Special:Contributions/141.101.98.188|141.101.98.188]] 00:33, 4 August 2015 (UTC)
:Yes the car would know that it was not permitted to drive in Alaska, however that would not prevent it from setting of with that destination in mind assuming that the person would take over control and entering manual driving in places where automatic were not permitted. [[User:Spongebog|Spongebog]] ([[User talk:Spongebog|talk]]) 01:03, 4 August 2015 (UTC)
::In that case, the "border" in the title text could be a state border... [[User:Sabik|Sabik]] ([[User talk:Sabik|talk]]) 06:13, 4 August 2015 (UTC)
:::But you are still assuming this is the present. If this comic is set in the future they could be anywhere in the US and drive all the way through the country, then through Canada and finally to the destination in Alaska. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 12:02, 4 August 2015 (UTC)
::::I think you mean North Montana, hasn't been called Canada in years [[Special:Contributions/162.158.255.156|162.158.255.156]] 16:59, 4 August 2015 (UTC)
:::::We always used to call Anchorage "North Seattle." Also, once we casually said "good morning" to a stranger on the street in Inverness (Scotland) and got the instant response "Vancouver!" We said "No, Seattle." The response to that was "Oh, same thing." [[User:Taibhse|Taibhse]] ([[User talk:Taibhse|talk]]) 10:49, 5 August 2015 (UTC)
:There's no reason to assume that the same laws apply in XKCD world, where velociraptors spontaneously attack people who use GOTO. There is also no direct indication that the self-driving car was being operated legally. I feel that trying to pin down the location of this comic is overthinking the matter. [[Special:Contributions/108.162.238.155|108.162.238.155]] 22:37, 5 August 2015 (UTC)
:I agree -- we're wasting valuable time that should be spent arguing that Black Hat is not in fact carrying a sand bag, but rather a large rock!

Explanation says that Black Hat expresses regret that the car won't make it to the border, and then speculates about his thoughts. However, only the title text mentions the border. [[User:Miamiclay|Miamiclay]] ([[User talk:Miamiclay|talk]]) 05:12, 6 August 2015 (UTC)

Talk:1545: Strengths and Weaknesses

2015-07-11T01:36:57Z

108.162.219.27:

For anyone who wants to take a stab at a more thorough (or better written) explanation of ancestry, the wiki pages for {{w|Identical_ancestors_point}} and {{w|Most_recent_common_ancestor}} helped me to start understanding the topic. I think its easy to jump to the conclusion that it is extremely unlikely that Cueball will be the ancestor of all living humans, however it isn't quite as intuitive as I believed. --[[User:Pudder|Pudder]] ([[User talk:Pudder|talk]]) 16:17, 1 July 2015 (UTC)

Look at the talk for that Wikipedia page. The figures for most recent common ancestor given there are incredible, but the community lacks the will to replace with decent science. [[User:Asimong|Asimong]] ([[User talk:Asimong|talk]]) 05:57, 2 July 2015 (UTC)

I don't understand, why would come a day that he is "either an ancestor to all living humans, or to none of them"? It's very possible for him to be the ancestor to ''some'' living humans forever [[User:Egoist|Egoist]] ([[User talk:Egoist|talk]]) 19:44, 1 July 2015 (UTC)
:If you have lots of children, and then if people interbreed randomly, as time goes forward it becomes less and less likely that any randomly selected person is ''not'' one of your descendents. As probability of ''not'' goes to zero, fraction of ''yes'' goes to 100%. But, if you do not have lots of children, and your kids don't either, at some future moment you may have zero descendents, and after that statistics cannot save you. [[User:Pesthouse|Pesthouse]] ([[User talk:Pesthouse|talk]]) 22:36, 1 July 2015 (UTC)

Two cases that haven't been dealt with:
1.) Humanity gets wiped out before this happens, so there are no living humans for Cueball to be ancestor to tor not. (0/0 case)

2.) Through physical separation, humanity diverges into 2 or more species, and Cueball is only an ancestor to some of these different species. Would all of these species be considered humans? I'm not familiar with the semantics.

I'm also not sure how likely either of these are.[[Special:Contributions/108.162.215.100|108.162.215.100]] 20:29, 1 July 2015 (UTC)
: In your case (1), that means he will *both* be the ancestor to all people or to none. [[User:MGK|MGK]] ([[User talk:MGK|talk]]) 01:21, 2 July 2015 (UTC)

Also! Assuming Cueball hasn't had kids yet, he is already an ancestor to no living humans. [[Special:Contributions/108.162.215.100|108.162.215.100]] 23:47, 1 July 2015 (UTC)

In this comic it is very likely that Cueball is confused regarding the software concept of recursion and "children" or child nodes. This plays off of the idea that if he reproduces he will eventually become a common ancestor to all existing humans at some point in the future (infinte recursion), or he won't reproduce and therefore will have no children (ancestor to none). This is further played with in the subtext which mentions implementing quick sort (a recursive sorting algorithm). Also of note, there are many other comics where cueball is confused by software concepts. [[Special:Contributions/108.162.219.111|108.162.219.111]] 21:11, 2 July 2015 (UTC)

I added a bit to this explanation. I think the explanation as it was dealt too heavily with the existential portion of the exchange and missed one really important part of the joke: Ponytail's questions are meant to be related to Cueball's PROFESSIONAL strengths and weaknesses (and his ambitions), but his responses are either accidentally or deliberately "missing the point" - he's responding from an existential point of view instead of a professional one. I hope that the portion of the explanation I added will help clarify this point. [[User:KieferSkunk|KieferSkunk]] ([[User talk:KieferSkunk|talk]]) 05:58, 4 July 2015 (UTC)

:Not only he's responding from existential point of view. Also, his answers are equally true for ANY interviewed.
:PS: I'm not sure why is sex described as "awkward". Sure, having sex during interview is very uncommon, but "awkward"? -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 14:01, 7 July 2015 (UTC)

We're fooling ourselves with job interviews, they are largely useless, and in fact may exclude the best candidates because they don't "interview well"; we would be better off doing a lottery after establishing basic credentials (literacy, basic communication ability...) Even Google has come to this conclusion, after all those puzzles and other crap:
:"Years ago, we did a study to determine whether anyone at Google is particularly good at hiring. We looked at tens of thousands of interviews, and everyone who had done the interviews and what they scored the candidate, and how that person ultimately performed in their job. We found zero relationship. It’s a complete random mess, except for one guy who was highly predictive because he only interviewed people for a very specialized area, where he happened to be the world’s leading expert." ( http://www.nytimes.com/2013/06/20/business/in-head-hunting-big-data-may-not-be-such-a-big-deal.html )

1545: Strengths and Weaknesses

2015-07-11T01:26:17Z

108.162.219.27: /* Explanation */

{{comic
| number = 1545
| date = July 1, 2015
| title = Strengths and Weaknesses
| image = strengths_and_weaknesses.png
| titletext = Do you need me to do a quicksort on the whiteboard or produce a generation of offspring or something? It might take me a bit, but I can do it.
}}

==Explanation==
[[Cueball]] is in a job interview and is being asked stereotypical job interview questions by [[Ponytail]], "What is your greatest weakness?", "What is your greatest strength?" and "Where do you see yourself in five years?"

In a roundabout way, Cueball answers that he is a living organism, and as such he has inherent flaws which could cause him to die. This is a reference to the fact that biological systems are "messy" and are not always optimal in design or operation. For example, cancer is a disease where the cellular machinery that governs cell replications breaks down and prolific cell division happens, endangering the organism through the creation of tumors. While this is a true weakness, it is also a weakness of all biological organisms and is not likely to help the interviewer determine if he is qualified for the job. However, it is likely to help the interviewer determine if he is right for the job -- because the interviewer is likely to stupidly presume that a person who gives silly and unhelpful answers is not right for most positions.

For the second question Cueball answers that he will one day be the ancestor to all living humans or none of them. As you go farther and farther into the future the ratio of people alive will either go to 0% or 100% of the descendants of the character. The {{w|Most_recent_common_ancestor|most recent common ancestor (MRCA)}} for humans is estimated to have been alive between 2,000 and 4,000 years ago. If the MRCA's ancestors are traced back, the {{w|Identical Ancestors Point}} can be found, at which point the entire population are either ancestors of all living humans or of no living humans. For humans, this point is estimated to be between 5,000 and 15,000 years ago.

In the last frame, for Cueball to be the ancestor to all living humans within 5 years means that all the humans who are not his children or grandchildren (including Cueball himself), must have died in a near total extinction of the human race - his apparent optimism about the possibility of this occurring would therefore be worrisome.

The overarching joke is that, rather than answer Ponytail's questions with answers relevant to the job she's interviewing him for, Cueball is answering her questions from an existential standpoint. He may be assuming that she wishes to assess his fitness as an organism from a genetic perspective (in which his biggest limitation is survival time and mortality), or he may simply be misinterpreting or deliberately avoiding her questions from a professional perspective.

The title text takes this further, equating producing offspring during an interview (which would be awkward for all involved) with something that may actually help assess a candidate's efficacy as an employee, namely writing out a sorting algorithm on the spot, another stereotypical interview question (see also [[1185: Ineffective Sorts]], especially the bottom left panel).

In [[1088: Five Years]], [[Beret Guy]] is also asked where he will be in five years, and he later interviews [[Hairy]] in [[1293: Job Interview]]. Other job interviews were portrayed in [[125: Marketing Interview]] and [[1094: Interview]].

==Transcript==
:[Ponytail is shown sitting on a swivel chair, to the left of a desk.]
:Ponytail: What would you say is your biggest weakness?

:[The view expands to show Cueball sitting on another swivel chair, on the opposite side of the desk.]
:Cueball: Probably that I'm a giant tangle of parts that don't always work right, so I can die easily.

:Ponytail: Biggest strength?
:Cueball: There will come a day when I'm either an ancestor to ''all'' living humans, or to ''none'' of them.

:Ponytail: Where do you see yourself in five years?
:Cueball: Probably not the ancestor of all living humans yet. But you never know!

{{comic discussion}}
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Cueball]]
[[Category:Job interviews]]