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Archiver > GENEALOGY-DNA > 2010-11 > 1290218417


From: Steven Bird <>
Subject: [DNA] Odds Are, It's Wrong - 5% of the time
Date: Fri, 19 Nov 2010 20:00:17 -0600
References: <F9C440A2-FC59-4A9E-AAAC-85DEE9D2FAB0@GMAIL.COM>, ,<COL115-W50D879F102DC3996D9D454A03A0@phx.gbl>,<COL118-W33D30F8531A4233CE0ED4EA03A0@phx.gbl>,<COL115-W265440F6CAC26EACE7E766A03B0@phx.gbl>
In-Reply-To: <COL115-W265440F6CAC26EACE7E766A03B0@phx.gbl>







The cumulative odds of losing three times in a row, given a 19 out of 20 chance to win on each play, are exactly what I stated. .05*.05*.05=.000125 or .00205% The odds of each chance remain 19 out of 20, but cumulative odds of winning ONE time increase. If I flip a fair coin, the chance of heads is 50/50. However, the chance of NO heads in ten tosses diminishes to 0.5^10, or .000977.



As I indicated before, if you are unhappy with a 95% chance of being right, then change the p value to 99.7% or 99.9999%. It is strictly up to the statistician. There is nothing sacred about p=0.05


You might be right about the double or nothing bet however. ;-)


> From:
> To:
> Date: Fri, 19 Nov 2010 23:56:12 +0000
> Subject: Re: [DNA] Odds Are, It's Wrong - 5% of the time
>
>
> Uh-oh - classic gambling fallacies.
>
> If you've lost twice already then the odds of losing three times in a row are just the odds of losing the third time. It doesn't get better the more you lose.
> And a double or nothing strategy works in favour of the party who has the last word on whether betting continues or not i.e. the bookmakers & casinos.
>
>
>
>
> > From:
> > To:
> > Date: Fri, 19 Nov 2010 17:36:31 -0600
> > Subject: Re: [DNA] Odds Are, It's Wrong - 5% of the time
> >
> >
> > The headline should have read:
> >
> > "Tom Sigfried fails to display any real understanding of statistics."
> >
> > 95% confidence interval means that you have a 1 in 20 chance of being wrong. If that isn't good enough for your purposes, then use a broader confidence interval, say 99.7% (3 sigmas). There's no mystery here.
> >
> > A steroids test that misidentifies 5% of the time is a bad test. Don't use it. However, if I have a 19 out of 20 chance of winning a bet, I'll take it. If I lose, I'll do it again double or nothing. Then the odds are now .05 *.05 that I will lose, or .0025. Third times a charm! Double or nothing at .05*.05*.05=.000125, or .0125% chance I'm wrong; about one chance in a thousand that I will lose three times in a row. I'll take those odds any time.
> >
> > Now, if I had a only 95% chance of making it home in my car tonight (5% chance of a fatal accident) I would probably consider postponing the trip to a more favorable time. :-) It all depends on your needs.
> >
> >
> > >
> > > Odds Are, It's Wrong: Science fails to face the shortcomings of statistics
> > > By Tom Siegfried
> > > Science News, March 27th, 2010; Vol.177 #7
> > > http://www.sciencenews.org/view/feature/id/57091/title/Odds_Are,_Its_Wrong
>
>
>
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