I need to create a flexible loop, which generates the regression of some variables that are not going to be constant over time, but depend on the number of follow-ups. I have the base matrix with the variable Weight, which will be completed in the following followings, and then I will separate it into different variables called Weight_ tracking number (Ex: Weight_1, Weight_2 ....). I need to create a previous loop so that it works independently of the number of variables that it has from the variable Weight and that later it can make a regression of each one of them. Can somebody help me?