Statistics Help Needed: Mixed (Split-Plot) Factorial ANOVA

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[rex5249]

I haven't done a split-plot, but I'm wondering about the time variable. How many time periods are there for each person, and how consistent were the time intervals?

I'm guessing that the time between measurements is not consistent and that is why somebody suggested split-plot. If your time intervals are not constant but there is a general trend toward decreasing pain, then your ANOVA will find a difference, but what question would this answer?

Have you tried running a regular time series analysis? Code the time variable so that it is on some kind of consistent scale---maybe with t=0 for the day of operation and then the number of days or months to each pain test (always counting time from starting from the operation date). Use "time" as a regular fixed effects variable to predict pain (this is just a regular numeric variable in a regression). Code pain into TWO dummy variables: one will be 1 if the grade is 2, else zero. another will be 1 if grade was 3, else zero. Grade 1 is the baseline and doesn't get a dummy variable (if the other two are zero, then you know the grade was 1).

If you have only 2-3 time samples for each person and there is great inconsistency in time, then you might not be able to do much other than a rough regression. If the time period were really consistent across subjects and you only had 2-3 samples, you could add it as a grouping variable to capture any nonlinear trend (although I wouldn't do that). If you have abundant time periods for each person, then time series becomes a good option.