Effect of survey size on student ratings of teaching

Questionnaires filled in by students have become increasingly important due to the part they play in decisions about the tenure, promotion and merit pay of faculty. Since it is rare to get a complete return, it is important to determine what reliance to place on various completion rates. Using data from the faculty of engineering in one university the authors conclude that if class size is 30 or more, a 50% response rate gives an acceptable indication of rating. In a small class, less than 30, something like an 80% return is needed if the same degree of confidence is to be achieved.