I have a dataset with a 2x2x2 design: Group (CTL vs. SC) x Session (Sess 1 vs. Sess 2) x Treatment (Treat vs. Placebo). The dependent variable is Response Time. I also have a variable Age. I would like to do an ANCOVA using Age as a covariate. To visualize data:
input <- mydata[,c("Group","Treatment","Session","ResponseTime","Age")] print(head(input)) Group Treatment Session ResponseTime Age <chr> <chr> <chr> <dbl> <dbl> 1 CTL Placebo Sess1 0.0126 53 2 CTL Placebo Sess1 0.0480 30 3 CTL Placebo Sess1 0.0318 58 4 CTL Placebo Sess1 0.747 28 5 CTL Placebo Sess2 0.150 28 6 CTL Placebo Sess2 0.0149 41 ...
Using example from this, I did the following:
options(contrasts = c("contr.treatment", "contr.poly")) model.1 = lm (Value ~ Group*Session*Treatment + Age + Group:Session:Treatment, data = mydata) library(car) Anova(model.1, type="II")
I'm unclear whether I placed Group where it belongs as it is a between- not within-subject variable. I think Age is used correctly as a covariate here but could someone take a quick look at let me know whether this is correct?