How to perform ANOVA for data measured with unequal time interval?

Question

So I have a set of patient data divided into 3 groups based upon their vision. I also have data on few variables that I believe to be relevent which are measured pre-surgery, 1 day post-surgery, and 7-day post-surgery. I am trying to figure out if those variables differ from each other statistically within and between those 3 groups, and a simple F-test should do the trick. However, since the measured time period had uneven time interval (pre-surgery - 1 day post-surger had shorter time interval than 1 day to 7 day). Is there any other test I should perform?

Also I am using R. I have read somewhere on how to deal with similar problem using SPSS, but the author was vague and didn't put any explanations on the reason behind. So if you guys can provide a simple procedure on how to perfom it using R it would be great !

Thanks a lot in advance !

Answer 1

You have a mixed design 3 x 3 two-way ANOVA. The time interval is in your case just an ordinal variable so it is irrelevant that the intervals are different. In R you can do a standard mixed ANOVA like this:

summary(aov(dv ~ group*surgery + Error(subject/surgery), data))

In case you violate any assumptions you can also conduct a robust mixed design two-way ANOVA like this:

library(WRS2)
bwtrim(dv ~ group*surgery, subject, data, tr = 0) 

# default is tr = 0.2 - 20% trimmed means
```