Hei, I have a time series for day and observation for a period of 365 days in variable a, a(:,1) being the daynumber (1 - 365) and a(:,2) the observations. I need to randomly select a day every week, so the first selection has to be a number from 1 - 7, the second day has to be a number between 8 - 14, etc. The selected daynumber have to be entered in a new variable (b). How is this done? Thanks for helping, Johannes.