Document ID: ./input/supremecourt_opinions/opinions/boundvolumes/558bv.pdf
Page Number: 290

Cite as: 558 U. S. 120 (2010) 

129 

Per Curiam 

in  the  panties—and  the  blood  sample,  the  likelihood  that  it 
is  not  Troy  Brown  would  be  .000033,”  id.,  at  460,  Romero 
ultimately agreed that it was “not inaccurate” to state it that 
way, id., at 461–462. 

Looking  at  Romero’s  testimony  as  a  whole,  though,  she 
also indicated that she was merely accepting the mathemati­
cal  equivalence  between  1  in  3  million  and  the  percentage 
ﬁgure.  At  the  end  of  the  colloquy  about  percentages,  she 
answered afﬁrmatively the court’s question whether the per­
centage  was  “the  same  math  just  expressed  differently.” 
Id.,  at  462.  She  pointed  out  that  the  probability  a  brother 
would match was greater than the random match probability, 
which also indicated to the jury that the random match prob­
ability  is  not  the  same  as  the  likelihood  that  someone  other 
than Troy was the source of the DNA. 

The  Mueller  Report  identiﬁes  a  second  error  in  Romero’s 
testimony:  her  estimate  of  the  probability  that  one  or  more 
of  Troy’s  brothers’  DNA  would  match.  Romero  testiﬁed 
there was a 1 in 6,500 (or 0.02%) probability that one brother 
would  share  the  same  DNA  with  another.  Id.,  at  469,  472. 
When  asked  whether  “that  change[s]  at  all  with  two  broth­
ers,”  she  answered  no.  Id.,  at  472.  According  to  Mueller, 
Romero’s  analysis  was  misleading  in  two  respects.  First, 
she  used  an  assumption  regarding  the  parents  under  which 
siblings  have  the  lowest  chance  of  matching  that  is  biologi­
cally possible, but even under this stingy assumption she re­
ported  the  chance  of  two  brothers  matching  (1  in  6,500)  as 
much  lower  than  it  is  (1  in  1,024  under  her  assumption). 
Second,  using  the  assumptions  Mueller  ﬁnds  more  appro­
priate,  the  probability  of  a  single  sibling  matching  respond­
ent is 1 in 263, the probability that among two brothers one 
or more would match is 1 in 132, and among four brothers it 
is 1 in 66.   Id., at 1583. 

In  sum,  the  two  inaccuracies  upon  which  this  case  turns 
are  testimony  equating  random  match  probability  with 
source  probability,  and  an  underestimate  of  the  likelihood