# Lying whore. **URL:** \ **Category:** The BBQ Pit\ **Created:** [April 10, 2006, 10:26pm UTC](https://boards.straightdope.com/t/lying-whore/352153 "2006-04-10T22:26:47Z")\ **Posts on this page:** 20\ **Page:** 52
**Author:** ![holmes](https://avatars.discourse-cdn.com/v4/letter/h/e95f7d/32.png) [@holmes](https://boards.straightdope.com/u/holmes)\ **Post date:** [May 30, 2006, 5:13pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1021 "2006-05-30T17:13:33Z")
> [@Weirddave](#): > > …still won’t change the fact that the crime she is claiming occured is statistically unlikely. It will just make the next one statistically more likely by a tiny amount. And that’s my problem. It’s statistically unlikely because of what? What makes it statistically unlikely? If the X factor that makes it statistically unlikely is present, then the statistic needs to reflect this. I get the impression that there’s this feeling that black skin is some sort of invulerabity against being ganged raped by white men. ---
**Author:** ![Pazu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pazu/32/15522_2.png) [@Pazu](https://boards.straightdope.com/u/Pazu)\ **Post date:** [May 30, 2006, 5:21pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1022 "2006-05-30T17:21:54Z")
> [@monstro](#): > > I predict that having **Pazu** post in agreement with **you with the face** , the idiots who are fighting with her will quiet down. This is probably due to my stupid typo in my first post, but I was really just trying to clarify things. Sensitivity and specificity are most useful in population-based studies (e.g. epidemiology). However, if you’re trying to translate population-based studies down to an individual level, the positive and negative predictive values are more relevant if you’re using Bayesian analysis. Which means that **Malacandra** ’s example is essentially correct. Let’s look at the stats problem he presented: > [@Malacandra's math problem](#): > > A witness to a hit-and-run identified the offending vehicle as a maroon car. Independent lab tests conducted with a balanced sample of images of red and maroon cars show that for every twenty times the witness identified a specimen as red, it was red nineteen times and maroon once; and that for every twenty times the witness identified a specimen as maroon, it was maroon nineteen times and red once. However, registration data confirms that, of all cars that are either red or maroon, only 2% are maroon. What is the probability that the witness is correct? (You are entitled to assume that only red cars and maroon cars need be considered in your answer. No variant on “A wizard did it” is acceptable.) From this presentation, we are given that: 1. The witness’s “sensitivity to maroon” is 95% (19/20 maroon cars identified as such). 2. The witness’s “specificity for maroon” is 95% (19/20 not-maroon [i.e. red] cars identified as such). 3. The prevalence of “maroon” is 2%. Let’s now use a population sample of 1000 cars. Of this population, 20 cars are maroon, and 980 are red. Of the 20 cars which are truly maroon, the witness will identify 19 of them as maroon, and 1 as not-maroon (red) with a sensitivity of 95%. Of the 980 cars which are truly not-maroon (red), the witness will identify 49 of them as maroon, and 931 as not-maroon (red) with a specificity of 95%. Therefore, the witness has identified a total of 19+49 = 68 cars as maroon, _but only 19 of them are truly maroon_. Thus, the positive predictive value of a “maroon” result is 19/68 = 27.9%. This is why general population screening for rare diseases is often found to be not worth the trouble, because the majority of positive results are false positives. I apologize in advance if I screwed up the numbers; it’s not really my field and I’m doing this off the cuff. ---
**Author:** ![Crotalus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crotalus/32/41_2.png) [@Crotalus](https://boards.straightdope.com/u/Crotalus)\ **Post date:** [May 30, 2006, 5:30pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1023 "2006-05-30T17:30:14Z")
> [@you with the face](#): > > Notice how my terms are different between the denominators, while yours are not. > > Before you define what a “true negative” is you need to define the question that’s being tested. On the basis of the hypothetical’s set up, that question reads “How accurately can this person identify a maroon car from a population from reds and maroon?” > > If the test was designed to show how accurately the person can ID red cars, the “needle in the haystack” would be reds. But it’s not, since the witness is being exposed to 49 times more reds than maroons. Are you following me up to this point? > > My point is that the 95% stat only applies to specificity but gives us no clue about the sensitity. As you can see from my formulas, the terms required to calculate sensitivity are unknown based on the info in the hypothetical. We don’t know how many maroons were correctly ID’d, and we don’t know how many were IDd incorrectly. > > \*We do know this term, my bad. It should be 49. Got carried away with the question marks. My formula terms Your formula terms True negatives reds identified as red True negatives reds IDed as red False positives reds IDed as maroons True positives maroons identified as maroon True positives maroons IDed as maroon False negatives maroons IDed as red These seem to be to be identical. What am I missing? Given the “givens” in the problem statement, it seems to me that all of the terms are known values. This is based on the assumption, which I may have incorrectly made, that the 95% accuracy applied to distinguishing red from maroon and maroon from red. On preview, I realize I have no idea how to make columns in a post. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 30, 2006, 5:33pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1024 "2006-05-30T17:33:59Z")
> [@Weirddave](#): > > …it still won’t change the fact that the crime she is claiming occured is statistically unlikely. It will just make the next one statistically more likely by a tiny amount. Let’s go with this for a minute. Let’s say, for the sake of argument, that only 5% of all rapes in the US are of White men raping Black women. How are you going to use that to determine if this case is credible unless you know something about the number of times that type of rape is falsely reported? What if only about 5% of all reported rapes are of Black women by White men? That would indicate that when a Black woman reports such a rape, it generally holds true-- even if the event itself is relatively rare. So, if you wanted to make some general statement about how believable your random White on Black rape is, you need to know something about how often false claims of rape are made in those circumstances-- which is something we don’t know. It’s not the rarity of the event that matters, because even rare events do happen (remember the 5% assumption). But let me be clear that even that type of statstical analysis is a gross oversimplification for all the reasons we’ve been talking about. Wwe realy don’t know what motivates your “average” rapist or whether the guys in question could be considered your “average” rape suspect. What is an “average rape suspect” anyway? ---
**Author:** ![Crotalus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crotalus/32/41_2.png) [@Crotalus](https://boards.straightdope.com/u/Crotalus)\ **Post date:** [May 30, 2006, 5:35pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1025 "2006-05-30T17:35:25Z")
> [@monstro](#): > > I predict that having **Pazu** post in agreement with **you with the face** , the idiots who are fighting with her will quiet down. It seems to me that the fools in this thread automatically disagree with her no matter what she says. Anyone else noticed this? > [@Pazu](#): > > Which means that Malacandra’s example is essentially correct. Superb analysis there, **monstro**. ---
**Author:** ![Zakalwe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zakalwe/32/270_2.png) [@Zakalwe](https://boards.straightdope.com/u/Zakalwe)\ **Post date:** [May 30, 2006, 5:41pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1026 "2006-05-30T17:41:19Z")
> [@you with the face](#): > > What we don’t know is how easy for the person to identify the 20 maroons. If they bat 100% on all the maroons, then it means very little if they slip up a few times on the reds. The witness is only credible if they can demonstrate skill in finding the needle in the haystack. Doesn’t it mean a lot in the hypothetical though? The witness has stated that the car in question is maroon. Based on the numbers provided, it’s actually much more likely that it’s red (regardless of what the witness said) and thus the police should continue to focus the majority of their investigation on red cars. Right? I’m also curious about your comment here: > [@](#): > > If you are saying that any given American has a 33% chance of being obese, you are wrong, as I already stated. You and I agree that we will pick 1 American citizen out of a voter registration roll. If that citizen is obese, you will pay me $5. If not, I will pay you $1. Who makes money and why? Does this not equate to a 33% that any given American citizen is obese? ---
**Author:** ![monstro](https://avatars.discourse-cdn.com/v4/letter/m/ba9def/32.png) [@monstro](https://boards.straightdope.com/u/monstro)\ **Post date:** [May 30, 2006, 5:44pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1027 "2006-05-30T17:44:40Z")
> [@Weirddave](#): > > Crotalus, Malacandra and YWTF herself are doing a fine job of ripping to shreads the idea that YWTF has any clue as to even simple mathmatical calculations. Here’s a tip to pass on to your sister: Everyone makes mistakes. Like your entry into this thread? Like your insistences that stats are relevant for determining the likelihood of the Duke accusation? Why should she admit to making mistakes she hasn’t made, when you can’t even concede that you are on the wrong side of an argument? > [@](#): > > When you make one as obvious as the one she made with the red/maroon car question, the correct response is NOT to despertly continue to blow smoke and obfuscate, it’s to say “Hey, you know what? I bungled that one. I was wrong. Sorry about that” and move on. But she DIDN’T bungle that. **Pazu** and even **Dragon Ash** both noticed the same flaw that **YWTF** caught. The only difference is that **YWTF** insisted it be acknowledged by **Malacandra**. And guess what? He DID. Why can’t you? > [@](#): > > If she’d done that it would have been over. Instead she has chosen to take the route of the holocaust deniers, despertly denying all evidence and looking like even more of a fool because of it. The only one in denial is you. _You_ know your argument stinks to heaven, but you’re too weak to admit it. > [@](#): > > And what does John Mace have to do with any of this? The stats remain relative as I have always stated they were: As one data point to be factored in when considering how likely the accuser’s story is to be true. What a minute. Earlier, didn’t you say that stats are useful only in the _absence_ of evidence. And now you say they are but “one data pointed to be factored in”. In with what? Evidence? If I have evidence that I got hit by a Ford Pinto, will stats showing that only 0.005% of the cars on the road are Ford Pintos need to be factored in to assess my allegation? I thought **Huerta’s** constantly overlooked point was that stats are helpful in the ABSENCE OF EVIDENCE. You dig your hole deeper whenever you post, **Weirddave**. > [@](#): > > Not as a determiner of her guilt or innocence, not as the sole data point, **not to be used in contradiction to actual evidence** , not to determine absolutely if her story is true or not, but simply as one data point indicating how likely her story is to be true. emphasis mine (Note that I’m quoting you directly so that I can’t be said to be “miscontruing” your posts) Can I ask you to clarify what the bolded part means? Because–and I might be wrong–it sounds like what you mean is that statistics should only be used when it corraborates, not _contradicts_, evidence. And if this is indeed what you’re saying, then you once again point to your own shoddy reasoning. Isn’t doing that cherry picking the data? > [@](#): > > Not all of your bombast and hysteria will change that, nor will other facts about this particular case. IF someone produces an unedited videotape tomorrow showing the accuser in this particular case being raped by the entire Duke lax team, Big Bird and Fidel Castro, while the Count Basie Orchestra plays _Stardust_ in accompanyment, and Jesus Christ desends from heaven to personally testify that he witnessed those events, it still won’t change the fact that the crime she is claiming occured is statistically unlikely. It will just make the next one statistically more likely by a tiny amount. Me being raped by a white man right now does not mean that a black woman in California is suddenly more at risk of being attacked by a white guy. Let me get the most elementary example I can think of: I have a bag full of marbles. **YWTF** , living thousands of miles away from me, has another bag of marbles. Both of our bags have nineteen white marbles, one red marble. The chance that either of us will draw out a red marble is 1/20. Mind you, this is set by the rules of probability, not a historical data set (aka _statistics_). I draw a red marble on my first attempt. What’s **YWTF’s** chances of drawing a red marble? 1/20. The same probability as it was before. This year, we might see a sudden increase in the incidents of white-on-black rapes, leading to a concomittant jump in the crime stats. This does not mean that black women are suddenly more at risk of being raped. Nor does it mean that an allegation of white-on-black rape is more plausible, more probable, or more likely. White rapists in Atlanta, GA are independent of white rapists in Suffolk County, NY, just like my red marble exists independently of **YWTF’s** red marble. And this hasn’t been asked yet, but what do you consider “likely”? Let’s say the crime stats say white teenagers represent 40% of those charged with shoplifting. Your teenage daughter is accused of shoplifting. Do you assume, based on the stats, that she’s a likely culprit? Does that particular statistic tell you anything about how many white teenagers are NOT shoplifters? Wouldn’t this be more important a statistic than the other one? Seriously, if you can’t see where your arguments fall short after this post, I will have to give up. ---
**Author:** ![you\_with\_the\_face](https://avatars.discourse-cdn.com/v4/letter/y/ecccb3/32.png) [@you\_with\_the\_face](https://boards.straightdope.com/u/you_with_the_face)\ **Post date:** [May 30, 2006, 5:50pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1028 "2006-05-30T17:50:41Z")
> [@Malandra](#): > > A witness to a hit-and-run identified the offending vehicle as a maroon car. Independent lab tests conducted with a balanced sample of images of red and maroon cars show that for every twenty times the witness identified a specimen as red, it was red nineteen times and maroon once; and that for every twenty times the witness identified a specimen as maroon, it was maroon nineteen times and red once. However, registration data confirms that, of all cars that are either red or maroon, only 2% are maroon. What is the probability that the witness is correct? I hope you can see that this hypothetical contains more meaningful info that the other that you presented. Using my formulas this is what I get: specificity = reds identified as red/( reds IDed as red+ reds IDed as maroons) = 19/(19+1) = .95 sensitivity = maroons identified as maroon/(maroons IDed as maroon + maroons IDed as red) = 19/(19+1) = .95 Which means that 95% of the time the witness should be able to correctly identify a maroon car. The prevalence of maroon cars is 2%. The formula for positive predictive value is: PPV = (maroon prevalence–MP)(sensitivity)/(MP)(sens) + (1-MP)(1-sens) = =(.02)(.95)/(.02)(.95) + (.98)(.05) = .28 or **28%** This is a relatively low number, so the witnesses testimony should be treated accordingly. Other evidence will be needed to pull up the slack, but they still would be good to put on the witness stand. Someone who can spot a maroon car correctly 95% is a reliable witness. As I hope is clear by now, **Malacandra** ’s orginal hypothetical did not provide the information necessary to reach this final conclusion. ---
**Author:** ![monstro](https://avatars.discourse-cdn.com/v4/letter/m/ba9def/32.png) [@monstro](https://boards.straightdope.com/u/monstro)\ **Post date:** [May 30, 2006, 5:52pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1029 "2006-05-30T17:52:01Z")
> [@Zakalwe](#): > > Doesn’t it mean a lot in the hypothetical though? The witness has stated that the car in question is maroon. Based on the numbers provided, it’s actually much more likely that it’s red (regardless of what the witness said) and thus the police should continue to focus the majority of their investigation on red cars. Right? > > I’m also curious about your comment here: > You and I agree that we will pick 1 American citizen out of a voter registration roll. If that citizen is obese, you will pay me $5. If not, I will pay you $1. Who makes money and why? Does this not equate to a 33% that any given American citizen is obese? I think you misunderstand her. If we have a stat showing that 33 % of Americans are obese, then yes–it will be safe to assume that a random draw of Americans will show that 33% of them are obese. But an _individual_ does not have a 33% _chance_ of being obese. A figure like this is based on an assessment of risk. One out of every four women has been sexually assaulted or abused. However, that doesn’t mean that women have a 25% chance of being raped. ---
**Author:** ![you\_with\_the\_face](https://avatars.discourse-cdn.com/v4/letter/y/ecccb3/32.png) [@you\_with\_the\_face](https://boards.straightdope.com/u/you_with_the_face)\ **Post date:** [May 30, 2006, 5:53pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1030 "2006-05-30T17:53:32Z")
> [@Crotalus](#): > > My formula terms Your formula terms > True negatives reds identified as red > True negatives reds IDed as red > False positives reds IDed as maroons > True positives maroons identified as maroon > True positives maroons IDed as maroon > False negatives maroons IDed as red > > These seem to be to be identical. What am I missing? Actually you’re right in that they are identical. Mea culpa. The problem comes in when you assume facts that are not in evidence such as the “number of maroons IDed as maroon”. ---
**Author:** ![Zakalwe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zakalwe/32/270_2.png) [@Zakalwe](https://boards.straightdope.com/u/Zakalwe)\ **Post date:** [May 30, 2006, 6:08pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1031 "2006-05-30T18:08:17Z")
> [@monstro](#): > > I think you misunderstand her. If we have a stat showing that 33 % of Americans are obese, then yes–it will be safe to assume that a random draw of Americans will show that 33% of them are obese. > > But an _individual_ does not have a 33% _chance_ of being obese. A figure like this is based on an assessment of risk. > > One out of every four women has been sexually assaulted or abused. However, that doesn’t mean that women have a 25% chance of being raped. Okay, I’ve got the obese thing. You’re right I did misunderstand the point. ---
**Author:** ![you\_with\_the\_face](https://avatars.discourse-cdn.com/v4/letter/y/ecccb3/32.png) [@you\_with\_the\_face](https://boards.straightdope.com/u/you_with_the_face)\ **Post date:** [May 30, 2006, 6:08pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1032 "2006-05-30T18:08:48Z")
> [@Zakalwe](#): > > Doesn’t it mean a lot in the hypothetical though? The witness has stated that the car in question is maroon. Based on the numbers provided, it’s actually much more likely that it’s red (regardless of what the witness said) and thus the police should continue to focus the majority of their investigation on red cars. Right? Not necessarily, if only for the fact that rare events do happen. This brings us back to the point that **Ellis Dee** was trying to argue about a murdered child. Just because a parent is statistically more likely to be the murderer than a stranger, it does not mean that any accusation that a stranger did it should be viewed skeptically. Particularly if there is a witness who can testify to that affect. The car thing is no different. A witness’s testimony is one data point, but it’s not enough by itself to bring a case down. A witness with 95% accuracy in IDing maroon cars is hardly something to dismiss, though. That doesn’t change just because maroon cares are rare. > [@](#): > > I’m also curious about your comment here: > You and I agree that we will pick 1 American citizen out of a voter registration roll. If that citizen is obese, you will pay me $5. If not, I will pay you $1. Who makes money and why? Does this not equate to a 33% that any given American citizen is obese? Your set up is equivalent to picking someone at random, which I already said should give you 33% odds. The problem would be if I was to say that every single American, regardless of underlying factors, has a 33% of being obese. This is not true at all. A young, nonsmoking, vegetarian male with an active lifestyle is not as likely to be obese as a middle-aged, smoking, omnivorous female with a sedentary lifestyle. ---
**Author:** ![Pazu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pazu/32/15522_2.png) [@Pazu](https://boards.straightdope.com/u/Pazu)\ **Post date:** [May 30, 2006, 6:09pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1033 "2006-05-30T18:09:57Z")
> [@monstro](#): > > But she DIDN’T bungle that. **Pazu** and even **Dragon Ash** both noticed the same flaw that **YWTF** caught. No, the “flaw” was that **neither** poster was evaluating the relevant statistic, which is the positive predictive value. The difference was that I thought it was pretty clear that **Malacandra** was getting at it, in a roundabout way. ---
**Author:** ![you\_with\_the\_face](https://avatars.discourse-cdn.com/v4/letter/y/ecccb3/32.png) [@you\_with\_the\_face](https://boards.straightdope.com/u/you_with_the_face)\ **Post date:** [May 30, 2006, 6:14pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1034 "2006-05-30T18:14:36Z")
> [@Pazu](#): > > No, the “flaw” was that **neither** poster was evaluating the relevant statistic, which is the positive predictive value. I could not even evaluate the PPV without sensitivity, and that was not assumable from the 95% statistic initial given. Is that clear to you? You already went on the record saying that this isn’t your area, so please watch how strongly you make your assertions. ---
**Author:** ![Crotalus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crotalus/32/41_2.png) [@Crotalus](https://boards.straightdope.com/u/Crotalus)\ **Post date:** [May 30, 2006, 6:16pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1035 "2006-05-30T18:16:46Z")
> [@you with the face](#): > > Actually you’re right in that they are identical. Mea culpa. The problem comes in when you assume facts that are not in evidence such as the “number of maroons IDed as maroon”. We’re getting clearer now! Cool. I made that assumption about the 95% applying to IDing both colors. I think that assumption was intended in the hypothetical. Maybe not, or maybe it was, but not clearly stated. I still think you would have been better off solving the simple math problem and then laying out the reasons it didn’t apply, rather than arguing against the structure of the problem first. That’s what I was reacting to (plus maybe a little bit of condescension of your part). I’m sorry if my choice of words offended you. That’s how I saw it at that moment, and we’re in the pit. Through the many pages of this thread, I’ve agreed (silently) with your position on the relevance of stats in the Duke case. ---
**Author:** ![DragonAsh](https://avatars.discourse-cdn.com/v4/letter/d/db5fbb/32.png) [@DragonAsh](https://boards.straightdope.com/u/DragonAsh)\ **Post date:** [May 30, 2006, 6:20pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1036 "2006-05-30T18:20:58Z")
> [@monstro](#): > > But she DIDN’T bungle that. **Pazu** and even **Dragon Ash** both noticed the same flaw that **YWTF** caught. Hey, what do you mean, _‘even’_? This might be the pit but let’s keep it civil. 😛 ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [May 30, 2006, 6:25pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1037 "2006-05-30T18:25:19Z")
> [@you with the face](#): > > I hope you can see that this hypothetical contains more meaningful info that the other that you presented. > > Using my formulas this is what I get: > > specificity = reds identified as red/( reds IDed as red+ reds IDed as maroons) > = 19/(19+1) = .95 > > sensitivity = maroons identified as maroon/(maroons IDed as maroon + maroons IDed as red) > = 19/(19+1) = .95 > > Which means that 95% of the time the witness should be able to correctly identify a maroon car. > > The prevalence of maroon cars is 2%. The formula for positive predictive value is: > > PPV = (maroon prevalence–MP)(sensitivity)/(MP)(sens) + (1-MP)(1-sens) = > > =(.02)(.95)/(.02)(.95) + (.98)(.05) > = .28 or **28%** > > This is a relatively low number, so the witnesses testimony should be treated accordingly. Other evidence will be needed to pull up the slack, but they still would be good to put on the witness stand. Someone who can spot a maroon car correctly 95% is a reliable witness. > > As I hope is clear by now, **Malacandra** ’s orginal hypothetical did not provide the information necessary to reach this final conclusion. Hoo-_rah_. Finally we get down to the meat and potatoes: that even a reliable witness’s report of a statistically unlikely event should be viewed with scepticism. And it’s not a bit of good putting the witness on the stand if counsel for the defence knows his math, for he can easily demonstrate that despite her 95% reliability in case of a balanced sample, the real-world figures are such that she is much more likely to be wrong than not. Of course, you can hope that the jury’s grasp of math is on a par with your own. The original hypothetical did provide the information necessary. It’s just that having got the answer wrong by being wedded to your notion that a 95% reliable witness must still be 95% likely to be right no matter what, you had to cast about you desperately to come up with a justification for your view. You even resorted to blurting out “Well, the witness could just say ‘red’ all the time and be right 980 times out of a thousand!”… without explaining how the witness thereby came to identify the car as maroon in the first place. :dubious: Well, finally we’ve managed to fight ignorance, and better late than never. Really, I wasn’t out to spring a “gotcha” on **you with the face** so much as to demonstrate a perfectly simple point: reports of unlikely events are probably mendacious or mistaken. Even if my “taste exact for faultless fact/Amounts to a disease”, the smart line to take if I start talking about the conversation I had this morning with a brontosaurus-riding Martian is to assume I’m not telling the truth, no? > [@monstro](#): > > **YWTF** , if you had never let anyone know you were a black woman, I think you would be getting a fairer shake around here. I know saying this is only adding fuel to an inferno (to which I say “Bring it on”), but I can’t think of any other reason why posters are giving you such a frickin’ impossible time here. Talking of “bring it on”, either produce substantive evidence that I am a racist or shove it up your stinkhole, cretin. The first four words after your last comma was where that sentence should have ended. :wally ---
**Author:** ![Pazu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pazu/32/15522_2.png) [@Pazu](https://boards.straightdope.com/u/Pazu)\ **Post date:** [May 30, 2006, 6:32pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1038 "2006-05-30T18:32:20Z")
> [@you with the face](#): > > I could not even evaluate the PPV without sensitivity, and that was not assumable from the 95% statistic initial given. Is that clear to you? You’re correct; I had simply assumed that his initial statement meant that the 95% applied both to sensitivity and specificity, but it’s true that there’s no reason to assume so. > [@](#): > > You already went on the record saying that this isn’t your area, so please watch how strongly you make your assertions. It isn’t my area, true; I’m not a statistician. I’m a pathologist, so I’m mostly familiar with clinical testing characteristics and how to translate them to clinicians who call and ask me questions like “The test was positive; how likely is it that my patient has disease X?” So can you explain to my simple mind why positive predictive value **isn’t** what we should be looking at? ---
**Author:** ![monstro](https://avatars.discourse-cdn.com/v4/letter/m/ba9def/32.png) [@monstro](https://boards.straightdope.com/u/monstro)\ **Post date:** [May 30, 2006, 6:39pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1039 "2006-05-30T18:39:46Z")
> [@Pazu](#): > > You’re correct; I had simply assumed that his initial statement meant that the 95% applied both to sensitivity and specificity, but it’s true that there’s no reason to assume so. > > It isn’t my area, true; I’m not a statistician. I’m a pathologist, so I’m mostly familiar with clinical testing characteristics and how to translate them to clinicians who call and ask me questions like “The test was positive; how likely is it that my patient has disease X?” > > So can you explain to my simple mind why positive predictive value **isn’t** what we should be looking at? Maybe it’s just me, but I don’t see how this is any shape or form relevant to the Duke rape allegation and the usefulness of stats in evaluating the merits of this case. We’re just naval gazing now. We were talking about population-level statistics like the DOJ crime stats. How did we get so far off track? We’re just naval-gazing now. Just trying to steer the conversation back to the original topic. ---
**Author:** ![you\_with\_the\_face](https://avatars.discourse-cdn.com/v4/letter/y/ecccb3/32.png) [@you\_with\_the\_face](https://boards.straightdope.com/u/you_with_the_face)\ **Post date:** [May 30, 2006, 6:40pm UTC](https://boards.straightdope.com/t/lying-whore/352153/1040 "2006-05-30T18:40:00Z")
> [@Malacandra](#): > > Hoo-_rah_. Finally we get down to the meat and potatoes: that even a reliable witness’s report of a statistically unlikely event should be viewed with scepticism. “Skepticism” is not the right word, though. Being skeptical suggests that something doesn’t sound right, e.g doesn’t sound reasonable or plausible, and _therefore_ should not be believed. The appropriate word to use in your hypothetical is “weight”. Meaning, how many weight do you give the witness’s testimony?. Is it damning evidence or is it relatively light-weight stuff? > [@](#): > > And it’s not a bit of good putting the witness on the stand if counsel for the defence knows his math, for he can easily demonstrate that despite her 95% reliability in case of a balanced sample, the real-world figures are such that she is much more likely to be wrong than not. This is a mistake. What lawyer in his right mind would turn away a witness who with 95% accuracy can identify a vehicle matching the defendant’s description? The PPV is relatively low, but the witness is still credible (reliable) and therefore, still adds evidence to the prosecution’s side. That’s all that is necessary. > [@](#): > > Really, I wasn’t out to spring a “gotcha” on **you with the face** so much as to demonstrate a perfectly simple point: **reports of unlikely events are probably mendacious or mistaken.** Your hypothetical doesn’t support this conclusion. To an extent, _all_ allegations are unusual and unique. That doesn’t necessarily make them false. \*\*There is nothing, on its face, improbable about a maroon car being involved in a hit-and-run. This doesn’t change if the maroon prevalence is 2% or 80%. \*\* I don’t know what else to say if you still disagree with me on this point. [Previous page](https://boards.straightdope.com/t/lying-whore/352153.md?page=51) [Next page](https://boards.straightdope.com/t/lying-whore/352153.md?page=53)