It is difficult to measure the pure union wage advantage because Multiple Choice women constitute a greater proportion of the workforce in strongly unionized industries than in weakly unionized industries. unionized industries tend to have larger plants that may require greater worker supervision, thus promoting union employers to seek out "superior" workers. unionized industries tend to employ production methods that are highly labor-intensive and therefore require lower-paid unskilled workers. unions are more easily established in those industries that pay low wages, so that the percentage wage gains are much smaller