Course review averages (per instructor) not weighted by class size

[jameslzhu]

When calculating average ratings for a professor over all instances of the same course (or all undergraduate courses), it is not weighted by class size. Thus low ratings for small-enrollment courses (i.e. CS 47A/B/C) disproportionately lower ratings for otherwise effective professors in large classes.

Example: https://hkn.eecs.berkeley.edu/coursesurveys/instructor/Ranade,Gireeja

The offending issue is in this line of code, which naively calculates the unweighted average of all course effectiveness ratings:

hkn-rails/app/views/coursesurveys/instructor.html.haml

Line 189 in 95e1a69

%td= rating_and_bar(numz.avg, courses.values.drop_while{|x|x.nil?}.collect{|x|x["#{s}_max".to_sym]}.compact.first || 1) unless courses.empty? || !numz.any?

As part of data cleanup, I've removed 0 ratings before for 0-response course surveys, but I think it's unwise to remove unusual (but non-zero) low course ratings, instead re-weighting them by their class size (which is usually small).