Measuring Survey Bias
In our recent Political Analysis paper (ungated authors’ version), Jocelyn Evans and I show how Martin, Traugott, and Kennedy’s two-party measure of survey accuracy can be extended to the multi-party case (which is slightly more relevant for comparativists and other people interested in the world outside the US). This extension leads to a series of party-specific measures of bias as well as to two scalar measures of overall survey bias.
Moreover, we demonstrate that our new measures are closely linked to the familiar multinomial logit model (just as the MTK measure is linked to the binomial logit). This demonstration is NOT an exercise in Excruciatingly Boring Algebra. Rather, it leads to a straightforward derivation of standard errors and facilitates the implementation of our methodology in standard statistical packages.
An Update to Our Free Software
We have programmed such an implementation in Stata, and it should not be too difficult to implement our methodology in R (any volunteers?). Our Stata code has been on SSC for a couple of months now but has recently been significantly updated. The new version 1.0 includes various bug fixes to the existing commands surveybias.ado and surveybiasi.ado, slightly better documentation, two toy data sets that should help you getting started with the methodology, and a new command surveybiasseries.ado.
surveybiasseries facilitates comparisons across a series of (pre-election) polls. It expects a data set in which each row corresponds to margins (predicted vote shares) from a survey. Such a dataset can quickly be constructed from published sources. Access to the original data is not required. surveybiasseries calculates the accuracy measures for each poll and stores them in a set of new variables, which can then be used as depended variable(s) in a model of poll accuracy.
Getting Started with Estimating Survey Bias
The new version of surveybias for Stata should appear be on SSC over the next couple of weeks or so (double check the version number (was 0.65, should now be 1.0) and the release date), but you can install it right now from this website:
net from http://www.kai-arzheimer.com/stata
net install surveybias
To see the new command in action, try this
use fivefrenchsurveys, replace
will load information from five pre-election polls taken during the French presidential campaign (2012) into memory. The vote shares refer to eight candidates that competed in the first round.
surveybiasseries in 1/3 , popvaria(*true) samplev(fh-other) nvar(N) gen(frenchsurveys)
will calculate our accuracy measures and their standard errors for the first three surveys over the full set of candidates.
surveybiasseries in 4/5, popvariables(fhtrue-mptrue) samplevariables(fh-mp) nvar(N) gen(threeparty)
will calculate bias with respect to the three-party vote (i.e. Hollande, Sarkozy, Le Pen) for surveys no. 4 and 5 (vote shares a automatically rescaled to unity, no recoding required). The new variable names start with “frenchsurveys” and “threeparty” and should be otherwise self-explanatory (i.e. threepartybw is $B_w$ for the three party case, and threepartysebw the corresponding standard error). Feel free to plot and model to your heart’s content.