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Idenitfying all the components of a design, again...

In my last post I talked about identifying all the components of a design. At least identifying them is an important step if we want to consider randomizing them. Of course, it's not necessary... or even feasible... or even desirable to do a full factorial design for every experiment. But it is still good to at least mentally list the potentially active components.

I first started thinking about this when I was doing a literature review for a paper on mixed mode designs. Most of these designs seemed to confound some elements of the design. The main thing I was looking for -- could I find any examples where someone had just varied the sequence of modes? The problem was that most people also varied the dosage of modes. For example, in a mixed mode web-telephone design, I could find studies that had web-telephone and telephone-web comparisons, but these sequences also varied the dosage. So, telephone first gets up to 10 calls, but telephone second gets 2 calls. Web first gets 3 email requests, web second gets 1 email request.

From such a design, I can't really say much about the sequence, as such. Maybe it doesn't matter much. Giving the same dosage might not make much difference for the effectiveness of the second treatment. On the other hand, it might.

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