How to plot two or more ROC curves on the same graph.

Prism's ROC (Receiver Operating Curve) analysis always analyses one set of data to create and graph one ROC curve. If you want to plot several ROC curves on one graph, you have to create them one at a time by rerunning the ROC analysis. To put several on one graph:

1. Go to the first ROC graph. 
2. Double click to bring up the Format Graph dialog.
3. Go to the middle tab. 
4. Click Add to add a data set to the graph, and pick the appropriate data set (the "ROC Curve" page of the appropriate ROC analysis. 
5. Repeat as necessary.

The idea here is that a single Prism graph can contain data sets from multiple tables. 

An alternative way to add a data set to a graph, is to grab the data set (the ROC results) in the navigator, and then drag and drop on to the graph.