Quantitative student evaluations of teaching (SET) and assessments are widely used in higher education as a proxy for teaching quality. However, SET are a function of individual rating behaviours resulting from student background, knowledge and personalities, as well as the learning experience being rated. SET from three years of data from a science department at a Russell Group University in the UK were analysed to highlight issues of sample size in relation to variable perceptions of modules, and develop a statistical model of feedback incorporating individual rating behaviours across modules. Key results are that sample size and individual rating behaviours have the potential to significantly affect summary module ratings, especially for <20 respondents or if individuals have heterogeneous views. A new approach is suggested, to interpret and compare quantitative module ratings, acknowledging uncertainty, variability and individual rating behaviours. This has implications for the interpretation of SET in many aspects of academic life, including university league table positions, the identification of good teaching practice with respect to student satisfaction, and the weight given to SET in individual academics' promotion applications.