I have a quantitative variable (medication dose) that is paired: dose of medication before surgery and after surgery for 76 patients.
I was under the impression that I should calculate the mean of the differences of the before and after doses and then calculate the standard error of the mean difference, CI and t-test. In SPSS a paired-sample t-test would do this.
I was told instead to do the percentage of the second dose (after surgery, i.e., 600 mg) as a function of the first dose (before surgery, i.e., 1200 mg) for every patient and then calculate the mean, standard error, CI and t-test of the percentages.
Is this a valid analysis?