Dear helpers in this forum,
I have the following question:
Suppose I have the following data set:
id x y
023 1 2
023 2 5
023 4 6
023 5 7
412 2 5
412 3 4
412 4 6
412 7 9
220 5 7
220 4 8
220 9 8
......
and i want to calculate sum_{i=1}^k
sum_{j=1}^{n_i}x_{ij}*y_{ij}
is there a simple way to do this within and between
subject summation in R?