When I drove home from work a couple of days ago, I noticed a policeman flagging down precisely every tenth car in the other lane and directing the drivers towards a lay-by. He was in uniform, wearing hi-vis gear and his government-issued Walther, so non-compliance was clearly not an issue. The scene was completed by a large billboard, stating that this was no ordinary vehicle spotcheck but rather a road use survey. I badly want these guys on our team.
In a recent paper, we derive various multinomial measures of bias in public opinion surveys (e.g. pre-election polls). Put differently, with our methodology, you may calculate a scalar measure of survey bias in multi-party elections.
Thanks to Kit Baum over at Boston College, our Stata add-on
surveybias.ado is now available from Statistical Software Components (SSC). The add-on takes as its argument the name of a categorical variable and said variable’s true distribution in the population. For what it’s worth, the program tries to be smart:
surveybias vote, popvalues(900000 1200000 1800000),
surveybias vote, popvalues(0.2307692 0.3076923 0.4615385), and
surveybias vote, popvalues(23.07692 30.76923 46.15385) should all give the same result.
If you don’t have access to the raw data but want to assess survey bias evident in published figures, there is
surveybiasi, an “immediate” command that lets you do stuff like this:
surveybiasi , popvalues(30 40 30) samplevalues(40 40 20) n(1000). Again, you may specify absolute values, relative frequencies, or percentages.
If you want to go ahead and measure survey bias, install
surveybiasi.ado on your computer by typing
ssc install surveybias in your net-aware copy of Stata. And if you use and like our software, please cite our forthcoming Political Analysis paper on the New Multinomial Accuracy Measure for Polling Bias.