T-test for percentage change Suppose I have some paired data (for example weight before and after), and I wish to find a confidence interval for the percentage increase in weight, can I simply apply the t-test to the percentage difference (A-B)/A or should I use another test?
 A: Percent change is an improper measure because of asymmetry.  If you really think that weight operates proportionally (it usually operates as a mixture of additively and proportionately) then analyze log ratio, which is a symmetric measure.  Get a confidence interval for that using standard methods, then anti-log to get fold change and its asymmetric confidence interval.  This fold change is a ratio of medians (also a ratio of means in this case, I think).
A: Using normally distributed dataset one can run a t-test for both, the absolute and the percentage change. Have a look here 
To keep things simple, if you apply a paired sample t-test for the previously computed %difference, i.e. $\frac{A-B}{A}*100$, then all you can do is a one-sample t-test which will test your new variable against a single value which I suspect would be 0 (0% change) using a one-sample t-test. 
You could instead run a paired-samples t-test (assuming that the assumptions of a t-test hold in your dataset) and then express the absolute mean difference which will be computed by any softare running a t-test as a %
