This study uses decision tree analysis to determine the most important variables that predict high overall teaching and course scores on a student evaluation of teaching (SET) instrument at a large public research university in the United States. Decision tree analysis is a more robust and intuitive approach for analysing and interpreting SET scores compared to more common parametric statistical approaches. Variables in this analysis included individual items on the SET instrument, self-reported student characteristics, course characteristics and instructor characteristics. The results show that items on the SET instrument that most directly address fundamental issues of teaching and learning, such as helping the student to better understand the course material, are most predictive of high overall teaching and course scores. SET items less directly related to student learning, such as those related to course grading policies, have little importance in predicting high overall teaching and course scores. Variables irrelevant to the construct, such as an instructor's gender and race/ethnicity, were not predictive of high overall teaching and course scores. These findings provide evidence of criterion and discriminant validity, and show that high SET scores do not reflect student biases against an instructor's gender or race/ethnicity.