I just recently had a requirement to randomize the response (DevTest's Think Time), between 1 - 2 seconds, and wanted to share the solution. Just change the (2) to whatever upper range is desired.
1. In the virtual service image (vsm), add a "Java Script Step" after "VS Image Response Selection". For Java code use:
Random rn = new Random();
int answer = rn.nextInt(2) + 1;
The 1st hit generated a think time 1.273 seconds. The 2nd 1.812, an so on ...