As you increase the cutpoint on the predicted probabilities from 0 to 1, the sensitivity decreases and the specificity increases. See the Classification Table (resulting from the CTABLE option) shown at the end of the example titled "Stepwise Logistic Regression and Predicted Values" in the LOGISTIC documentation. By plotting sensitivity against 1-specificity, the point at which sensitivity is zero can be placed at the lower left corner of the plot since 1-specificity is also zero. Similarly, when sensitivity=1, 1-specificity is also 1 so that point can appear in the upper right corner of the plot. ... View more