This paper reported an investigation into the non-response cases in student evaluation of teaching (SET). Data were collected through an online system called COSSET during regular SET exercises and through specially designed questionnaire surveys. Two kinds of response rates were studied. One is sectional (class) response rate which is the percentage of students in a section (class) who successfully complete the SET questionnaire. The other is individual student response rate which is the percentage of SET questionnaires completed by an individual student in a single SET exercise. Results showed that sectional response rates are affected by various factors relating to the data collection process and students' background. Individual student response rates are affected by various factors including students' background and attitudes.