I have 40 people that I measure at baseline, getting their mean level of X at time zero. I also calculate the standard deviation and standard error of the mean of X.
Then after 100 days I measure their levels of X again, and again calculate a mean, standard deviation, and standard error. I lost five people to follow up, so the N for this group is only 35.
Time N MeanX SD SE 0 40 6.9 5.2 0.8 100 35 5.7 5.7 1.0
I am interested in the difference in mean levels of X between 100 days and baseline. So I can calculate this easily, as
5.7 - 6.9 = -1.2
My question is... For this value, -1.2, I would also like to know its standard deviation and standard error. Could someone tell me how to do this? I've found a few possible formulas on the internet, including one from a similar question on this site, for example squaring the standard deviations, dividing them by their n's, and then taking the square root, but I am not one hundred percent sure if this is what I want.