What is the standard error of d ? 

Suppose we decide a better measure of the effectiveness of the drug is the mean decrease in blood pressure rather than the measure of effectiveness used previously. Let d_i = x_i − y_i ,i = 1, . . . , 100, where xi = DBP for the ith person before taking the drug and yi = DBP for the ith person 1 month after taking the drug. Suppose the sample mean of the di is +5.3 and the sample variance is 144.0.