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 the computing system; (b) automatically generating at least one first utterance from a first machine text to speech system for said candidate challenge sentence using the computing system, said at least one first utterance including first acoustical characteristics; (c) automatically generating at least one second utterance from a human speaker for said candidate challenge sentence 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 storing said at least one first utterance and said at least one second utterance and said candidate challenge sentence in a challenge item database using the computing system for use by an utterance based challenge system when said candidate challenge sentence acoustic score exceeds a target threshold.