During the recent debates over public employees’ collective bargaining rights, especially around the Wisconsin protests, I heard a few people argue that Republican governors are intent on destroying public sector unions, at least in part, because union members are more likely to vote – and to vote Democratic.
The latter argument (union members are more likely to vote Democratic) is generally true (also here) – although the union "effect" on candidate/party choice is of course complicated. The former argument (more likely to vote in general) is also valid, but there is some underlying public/private variation that is both interesting and important.
As is almost always the case, isolating the effect of a given factor (in this case, how being a union member affects the likelihood of voting) requires one to compare how this factor “operates” on people who are otherwise similar. For example, in a previous post, I compared public and private sector workers’ earnings. In order to uncover the “effect” of public sector employment on earnings, I used models that controlled for other relevant, measurable factors, such as education and experience. In doing so, I was able to (imperfectly) ensure that I was comparing public and private employees who were similar in terms of skills and qualifications.
The same basic concept applies to voting.