From: beaver@castor.cs.psu.edu (Don Beaver)
Newsgroups: misc.legal,alt.dads-rights,soc.men
Subject: Re: NO SUBJECT (1)
Date: 30 Aug 92 16:10:05 GMT

In article <1992Aug30.145055.4741@wolves.uucp> woody@wolves.uucp (Woody Muller) writes:
>In <1992Aug29.032604.28702@zooid.guild.org> goid@zooid.guild.org (Will Steeves) writes:
>
>>James, from what I know, a simple blood test will only detect inconsistencies
>>in blood types between father, mother, and child (I *think* that the mother's
>>blood *is* needed ... Can SOMEONE please confirm whether this is correct or
>>not - Very Important), and therefore can only tell you whether you are *not*
>>the father.
>
>Yes, a blood test requires samples from father, mother, and child.  The
>Paternity Evaluation Report I just pulled from my records (from early 1990)
>columnates about 10 separate tests and assigns a probability factor
>they call a "paternity index."  They multiply the 9 or 10 probabilities
>into an "aggregate paternity index."
>
>================ [document text portions]
[various data deleted]
>________________________________________
>                                   Aggregate = 1461:1
>
>CONCLUSIONS:
>"The alleged father, [me], cannot be excluded as the biological father
>of [Peter].  Based on the above genetic testing results, the probability
>of paternity is 99.93%, as compared to an untested random man of the
>North American Caucasian population.  (Prior Probability = .5)"
                                        ^^!!!!!!!!!!!!!!!!!!^^

"Probability of paternity" is a miscomputed and misleading measure.

From Ellman and Kaye, "Probabilities and Proof: Can HLA and Blood
Group Testing Prove Paternity?" in Foote, Levy, Sander, 
"Cases and Materials on Family Law" (1985), 774ff:

  There seems to be no basis for the blanket probability that
  the prior probability is one-half.  ...  For instance, if
  50% of paternity cases are decided for defendants, one might think
  that the prior probability should be taken to be 0.50.

  ... Indeed, even had the defendant in Cramer shown that he was
  imprisoned in another state during the critical period,
  the expert's calculation of a 98.3% chance of paternity would
  not have been affected.


The "probability of paternity" is based on Bayesian analysis,
which judges and laypeople alike don't tend to understand well.
As Ellman and Kaye point out:

  [The 0.50 prior probability] is equivalent to supposing that the
  universe of fathers is already reduced to two equally likely
  suspects before considering the HLA test results.

  As the application of Bayes' Theorem shows, making such an assumption
  ensures that in every case in which any recognized blood test does not
  exclude the defendant, one will find that he is probably the father,
  thus satisfying the burden of proof in civil cases.


Given the lack of full understanding by those who hear the claims
of experts -- even when they make their assumptions explicit
("Prior Probability = 0.5") -- I'd be a bit careful about claims 
of +positive+ ID, even though they +are+ used in court.

On the other hand, these tests are pretty conclusive when they
have a +negative+ ID.

So the answer is: if you want to show paternity, don't delve into the 
details of a positive ID, and if you want to disprove paternity, 
make sure the judge knows the objections and implicit assumptions
to a positive ID.

Don
-- 
beaver@cs.psu.edu                       Opinions from the PC-challenged
Newsgroups: misc.legal