I tried it out and there's clearly a problem with the random number generator. All subtraction results were around 5 or 50. There was some variation in them -- some 51's and some 45's and 46's, but no variation from that in about 30-40 trials. If it's written in Cocoa, you might want to try my RandomSequence Obj-C class, a thread-safe wrapper for the random(3) system call.