Date: Wed, 2 Aug 2000 09:46:59 -0400 Reply-To: "Sorkin, John" Sender: "SAS(r) Discussion" From: "Sorkin, John" Subject: Multiple t-tests and Type-I error Comments: To: "J.Groeneveld@ITGROUPS.COM" Comments: cc: "patrickm@MAIL.NEWCASTLE.EDU.AU" Content-Type: text/plain; charset="iso-8859-1" Jim, Please help me clarify my thinking. You suggest that adjustment for multiple comparisons is only necessary when performing a series of non-independent tests. I believe your example was a-b, b-c, c-a. In this case, I agree as the third comparisons is a linear combination of the first two comparisons: c-a=-(a-b + b-c). You also state that in the case of a series of independent comparisons, a-b, a-c, a-d, a-e no adjustment is needed. I was taught that adjustment for multiple comparisons is needed when performing multiple independent tests as the probability of a type-I 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 type-II 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.

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