Date: Mon, 4 Nov 1996 10:19:59 -0500 "William B. Ware" "SPSSX(r) Discussion" "William B. Ware" Re: Simple mixed design To: Jennifer Marshall <96Nov4.091442-0500_est.15775-149496+180@email.unc.edu>

On Mon, 4 Nov 1996, Jennifer Marshall wrote:

> I am doing a simple mixed design with one between subjects factor and one > within subjects factor.I have gotten an error term for the between > subjects factor and for the interaction of the within and between subject > factors. I do not understand where the error term comes from. > Could someone please explain the error term AND where it comes from?

Here is the way that I conceptualize the design, as I understand you. Let's call the between-factor A and the within-factor B. You have observations nested within the A factor, but crossed with the B factor. You should have two error terms: a between and a within. The "between" error term is simply the pooled error term within each level of A. That is, there are cases within A1, cases within A2, etc. It works just like a oneway design. The "within" error term serves for both the B effect and the A*B effect.

Think of the "within" term this way. Within each level of A, you have a randomized block design; cases by B. Within each of those designs, you have a B effect, a subjects effect, and a residual. The overall "within subjects" error term is the pooled residuals...

In testing B and A*B, you need to attend to some assumptions. First, that you have homogeneity of residuals, and that the pooled variance-covariance matrix is "symmetric." You might want to look at Roger Kirk's book on Experimental Design.

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William B. Ware, Professor and Chair Educational Psychology CB# 3500 EMAIL: wbware@unc.edu University of North Carolina PHONE: (919)-966-5266 Chapel Hill, NC 27599-3500 FAX: (919)-962-1533

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