Jocelyn Evans and I have developed a method for calculating such a figure by extending Martin, Kennedy and Traugott’s measure to the multi-party case. Being the very creative chaps we are, we call this new statistic [drumroll] . We also derive a weighted version of this measure , and statistics to measure bias in favour/against any single party (). Of course, our measures can be applied to the sampling of any categorical variable whose distribution is known.
We fully develop all these goodies (and illustrate their usefulness by analysing bias in French pre-election polls) in a paper that (to our immense satisfaction) has just been accepted for publication in Political Analysis (replication files to follow).
Our module survebias is a Stata ado file that implements these methods. It should become available from SSC over the summer, giving you convenient access to the new methods. I’ll keep you posted.