ds = dataset you are currently using.
DV = dependent variable of interest
IV = independent variable of interest
Subject = name of subject number column
XYXY = dummy name for a variable, matrix, or data frame into which you are moving information.
You have several trials of a dependent variable and you would like to get the mean scores at different levels of the independent variables.
Note: "DV1", "DV2", "IV1", "IV2" are names that you are giving to the columns (variables) in the aggregated data set. "DVa", "DVb", "IVa", "IVb" refer to the names in the data set you are collapsing.
XYXY.agg <- aggregate (list (DV1 = ds$DVa, DV2 = ds$DVb),
by = list (Subject = ds$Subject,
IV1 = ds$IVa,
IV2 = ds$IVb),
FUN = mean, na.rm = TRUE)