When we are evaluating a tool, we evaluate it against its design specifications; mathematics and deduction tell us from there that it will produce its intended outcome. It is only when we evaluate the use of a tool that we evaluate against the actual outcome. So measuring drop-out rates, counting test scores, and adding up student satisfaction scores will not tell us whether a MOOC was successful, only whether this particular application of this particular MOOC was successful in this particular instance.
Read the full text - download available. Issue is MOOC as a "tool" for educators and learners, not as an end in itself - so assessment strategies must be above and beyond test scores, satisfaction and completion rates.