1. A method embodied in a computer readable medium for generating challenge data to be used for accessing data and/or resources of an electronic computing system comprising: (a) automatically generating a candidate challenge sentence from a first set of words and phrases using a computing system; (b) automatically generating at least one first utterance from a first machine text to speech system of said candidate challenge sentence using the computing system, said at least one first utterance including first acoustical characteristics; (c) acquiring at least one second utterance of said candidate challenge sentence known to be from a human speaker using the computing system, said at least one second utterance including second acoustical characteristics; (d) automatically determining a difference in said first and second acoustical characteristics using the computing system to determine a challenge sentence acoustic score for said candidate challenge sentence; (e) automatically selecting said candidate challenge sentence as a suitable challenge item in an utterance-based challenge system and storing said at least one second utterance and said candidate challenge sentence in a challenge item database using the computer system when said candidate challenge sentence acoustic score exceeds a target threshold, indicating that said at least one first utterance from the first machine text to speech system is sufficiently different from said at least one second utterance from the human speaker for said candidate challenge sentence to be used to automatically distinguish between humans and machines attempting to access the electronic computing system.