Hello I have a problem which I need to multiply X1 by 100 weights W1 ... W100 and then sum them (weighted sum). My dataset has 100,000 cases So I decided to compute matrix products in order to avoid loops.   Here is my code:   data datos; input group $ x1 x2 w1 w2 w3; datalines; A 55.3 41.1 2 3 1 A 50.5 40 3 2 2 A 52.4 39.8 2 1 3 A 58.1 39 1 2 1 A 54 39.5 2 3 2 A 55.6 40.5 3 2 3 B 49.5 50.5 5 5 5 B 48.8 50 4 6 6 B 50.1 49.5 6 4 5 B 49.9 48 5 5 5 C 60.1 60 8 9 6 C 59.9 61 7 8 7 C 58 59 8 7 8 C 58.5 60.5 7 8 8 C 61.2 59.5 8 7 9 C 60 60 9 8 9 ; run; %LET x = x1 x2; %LET w = w1 w2 w3; proc iml; use datos; read all var { &x} into x[rowname=group colname = nr]; read all var { &w} into w[rowname=group colname = nc]; close datos; mattrib a rowname = nr colname = nc label = "Overall sum"; a = t(x)*w; print a; quit; I currently obtain this: Overall sum   w1 w2 w3 x1 4495.8 4501.4 4489.4 x2 4329.3 4332.6 4338.5 but now I need to calculate matrix product by group in order to get this result: sums by group     w1 w2 w3 A x1 699.8 708.7 646.4 A x2 521.3 520.6 480.0 B x1 992.8 990.2 1040.3 B x2 989.5 990.5 1040.0 C x1 2803.2 2802.5 2802.7 C x2 2818.5 2821.5 2818.5 Thanks ... View more