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Page Number: 21.0

18 

SMITH v. ARIZONA 

Opinion of the Court 

and whether her results should be trusted.  In short, Ari-
zona wants to end run all we have held the Confrontation 
Clause to require.  It cannot. 

Properly understood, the Clause still allows forensic ex-
perts  like  Longoni  to  play  a  useful  role  in  criminal  trials.
Because Longoni worked in the same lab as Rast, he could
testify  from  personal  knowledge  about  how  that  lab  typi-
cally functioned—the standards, practices, and procedures 
it used to test seized substances, as well as the way it main-
tained chains of custody.  (Indeed, Longoni did just that in 
a different part of his testimony.  See App. to Pet. for Cert. 
32a–39a.)  Or had he not been familiar with Rast’s lab, he 
could have testified in general terms about forensic guide-
lines  and  techniques—perhaps  explaining  what  it  means
for a lab to be accredited and what requirements accredita-
tion imposes.  Or as the Williams plurality and dissent both 
observed,  he  might  have  been  asked—and  could  have  an-
swered—any number of hypothetical questions, taking the 
form of: “If or assuming some out-of-court statement were 
true, what would follow from it?”  See 567 U. S., at 67–68; 
id., at 129, n. 2.  (The State of course would then have to
separately  prove the thing assumed.)  The United States, 
appearing as amicus curiae in support of neither party, use-
fully  addressed  these  matters  at  oral  argument,  distin-
guishing  Longoni’s  testimony  as  block-quoted  above  from
the  various  kinds  of  testimony  just  described.    See  Tr.  of 
Oral Arg. 36–41.  The latter forms of testimony allow foren-
sic expertise to inform a criminal case without violating the 
defendant’s  right  of  confrontation.  And  we  offer  these 
merely as examples; there may be others. 

But as the United States acknowledged, the bulk of Lon-
goni’s testimony took no such permissible form.  Ibid.  Here, 
the  State  used  Longoni  to  relay  what  Rast  wrote  down 
about  how  she  identified  the  seized  substances.    Longoni
thus effectively became Rast’s mouthpiece.  He testified to 
the precautions (she said) she took, the standards (she said)