ab.news(a)laposte.net @ INTERNET skrev 2007-09-18 00:13:18 : I got the same kind of error for the test statistic calculator for test of 2 proportions, but the pop-up says "Bad PNG header: Got bytes fc e0 22 0" instead. Also, no graph is produced. BTW, if one by mistake inputs a number instead of a proportion, one gets the following weird output: Null hypothesis: the population proportions are equal Sample 1: n = 450, proportion = 10 Sample 2: n = 520, proportion = 12 Test statistic: z = (10 - 12) / -1.#IND = -1.#IND Two-tailed p-value = 1.#QO (one-tailed = 1.#QO) I guess -1.#IND and 1.#QO are supposed to mean NA. This is the Windows snapshot of version 1.6.6pre1 build date 09/17/2007. Andreas