Date: Wed, 2 Aug 2000 09:46:59 0400
ReplyTo: "Sorkin, John" <SorkinJ@GRC.NIA.NIH.GOV>
Sender: "SAS(r) Discussion" <SASL@LISTSERV.UGA.EDU>
From: "Sorkin, John" <SorkinJ@GRC.NIA.NIH.GOV>
Subject: Multiple ttests and TypeI error
ContentType: text/plain; charset="iso88591"
Jim,
Please help me clarify my thinking. You suggest that adjustment for multiple
comparisons is only necessary when performing a series of nonindependent
tests. I believe your example was
ab, bc, ca.
In this case, I agree as the third comparisons is a linear combination of
the first two comparisons:
ca=(ab + bc).
You also state that in the case of a series of independent comparisons,
ab, ac, ad, ae
no adjustment is needed.
I was taught that adjustment for multiple comparisons is needed when
performing multiple independent tests as the probability of a typeI error
when performing N tests is N*0.05 assuming the tests are preformed at a 5%
significance level. (To simplify the disucssion I am ignoring the
probability of making a typeII error.) Have I missed something?
Thank you,
John Sorkin
P.S. I have sent a copy of this message to Patrick. I invite Patrick to help
me clarify my thinking.
