Student evaluations of teaching are widely adopted across academic institutions, but there are many underlying trends and biases that can influence their interpretation. Publicly accessible web-based student evaluations of teaching are of particular relevance, due to their widespread use by students in the course selection process and the quantity of data available for analysis. In this study, data from the most popular of these websites, RateMyProfessors.com, is analysed for correlations between measures of instruction quality, easiness, physical attractiveness, discipline and gender. This study of 7,882,980 RateMyProfessors ratings (from 190,006 US professors with at least 20 student ratings) provides further insight into student perceptions of academic instruction and possible variables in student evaluations. Positive correlations were observed between ratings of instruction quality and easiness, as well as between instruction quality and attractiveness. On average, professors in science and engineering disciplines have lower ratings than in the humanities and arts. When looking at RateMyProfessors as a whole, the effect of a professor's gender on rating criteria is small but statistically significant. When analysing the data as a function of discipline, however, the effects of gender are significantly more pronounced, albeit more complex. The potential implications are discussed.