Hi! To solve a problem like A*x=c where A is a matrix (n*n) and x and c are two vectors (n*1), one should ALWAYS use the SOLVE function. If n is big there will be a HUGH difference in computing time and resources but most important in accuracy. To calculate x as INV(A)*c requires n more operations, takes more resources (at least n times more) and thus the accuracy is strongly decreased. Of course if n is small you will not notice this, since SAS software use well designed solutions to this classic problem in linear algebra. This problem is often used in teaching students in Numerical Analysis. Another point is that the inverse of A usually is of little interest as such. I have seen it in a few advanced applications in mathematical statistics. But that will only matter if you program deep statistical methods yourself. These problems are usually solved in a nice and standardized way by SAS Statistical Procedures. / Br Anders Skollermo Ph.D. Computational Physics