These are centered discrete change coefficients. Suppose that you fix the level of x_k at its mean. The discrete change labels -+1/2 indicates the change from the mean(x_k)-1/2 to mean(x_k)+1/2 holding other variables at their fixed values. Similarly, -+sd/2 indicates a change from mean(x_k)-1/2*sd(x_k) to mean(x_k)+1/2*sd(x_k). Also take a look at the options uncentered and delta() for prchange. (April 4, 2001)