An Evaluation of Course Evaluations. Student ratings of teaching have been used, studied, and debated for almost
a century. This article examines student ratings of teaching from a
statistical perspective. pdf 
Since 1975, course evaluations at University of California, Berkeley have asked: Considering both the limitations and possibilities of the subject matter and course, how would you rate the overall teaching effectiveness of this instructor?
We don’t measure teaching effectiveness. We measure what students say, and pretend it’s the same thing. We calculate statistics, report numbers, and call it a day.
If we want to assess and improve teaching, we have to pay attention to the teaching, not the average of a list of student-reported numbers with a troubled and tenuous relationship to teaching. Instead, we can watch each other teach and talk to each other about teaching.
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As with Big Data we struggle to coax a weak signal out of noise when the strong signal is there for the asking. May I sit in on your class today to observe?