The "random" index for atRandom is, basically, computed prngOutput mod: selectionRange. For large selection ranges, it fails because the period of the PRNG used by atRandom is 2^30th, with the output an integer in the range 1 - 2^30th. (in other words, the prng is a repeating sequence of every number between 1 and 2^30th) That means: - There will be bias when the range you pick from approaches 2^30th. - There will be gaps (indexes that won't ever be picked) once you pass 2^30th. The reason is techical; the default RNG is a reasonable compromise between speed, memory, and usable range. If the intended use requires a larger range, an appropriate random number generator can be provided using atRandom: It certainly could give an error and/or select another RNG automatically if outside applicable range, but no one has been bothered enough to do either, yet. Cheers, Henry -- Sent from: http://forum.world.st/Pharo-Smalltalk-Developers-f1294837.html