I have a variable, on a continuous scale, it is some medical value before getting a treatment. After the treatment, this value is being reduced. For some people by 10%, for some by 13%, 15%, 20%, etc,....

I have two of these treatments, and I am comparing them. Is it legal (I can't think of a reason why not) to compare the mean reduction ? By mean reduction my intention is: if in one group there are 3 patients (just for the example), and the first had reduction of 10%, the second 13% and the third 17%, than I can say that the average reduction was 13.33%. Now I want to compare the mean reduction for both groups. I make a mean of percentages, is it legal ?

thanks