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Date:         Wed, 22 Oct 2003 11:21:04 -0400
Reply-To:     Peter Flom <flom@NDRI.ORG>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Peter Flom <flom@NDRI.ORG>
Subject:      Re: Missing data in Proc Mixed
Content-Type: text/plain; charset=US-ASCII

At 07:49 22/10/03 -0600, William Dudley wrote: <<< I am responding to a reviewer about the use of proc MIXED in a repeated measures ANOVA with treatment and control.

We think that we are employing an Intent To Treat method because Proc mixed does not employ listwise deletion as does proc GLM. However the reviewer thinks that Proc Mixed must be substituting values. I notice that most authors simply say that because Proc Mixed using ML estimation,that it employs all of the data and cases are not eliminated if they provide at least one data point. >>>

I think the reviewer is correct, at least in substance if not in detail.

As far as I know, Intent to treat refers to the design of the study, that is, you assign people to Treatment or Control, and then, even if someone in the Treatment group doesn't 'do' the treatment, he/she is still part of the Treatment group. Thus, you'd be measuring 'what really happens' instead of 'what the treatment does'; it seems to me to be an attempt to increase external validity at the cost of internal validity.

What PROC MIXED does is different, and is, indeed one of the strengths of HLM. It uses all the data that's available. Thus, if you schedule 4 followups, and a person goes to only 1 of them, it uses the information from that one followup. Whether this means it 'substitutes values' depends, I suppose, on what you mean by 'substitutes values'; but I don't think it could be called an 'intent to treat' model.

HTH

Peter L. Flom, PhD Assistant Director, Statistics and Data Analysis Core Center for Drug Use and HIV Research National Development and Research Institutes 71 W. 23rd St www.peterflom.com New York, NY 10010 (212) 845-4485 (voice) (917) 438-0894 (fax)


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