This article has an almost-decent sample size and the sort of conclusion that magazines like the Chronicle love to publish: "Data was collected via 516 responses to an online survey and achievement tests.... The credit bearing group also scored significantly higher achievement scores than the credit careless group. Credit clearly and significantly affected all dependent variables investigated in this study." Now of course I'd like to see more courses studied (the only one here is 'Ataturk's Principles and History of Turkish Revolution' which while no doubt interesting (I'd take it) is a bit niche). And the sample (which "consisted of faculty of education students who responded to the online survey") appears to be dangerously self-selective.
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