There is much variation in the union/nonunion wage gap across groups of workers within each of the two sectors, public and private. Furthermore, the variation in the public sector does not parallel in all of its detail that in the private sector. Thus, though the public-sector gaps typically are somewhat below their private-sector counterparts, there are important exceptions to this difference, especially among employees of local governments: public school teachers, clerical workers, refuse collectors, local transit bus drivers, licensed practical nurses, hospital technicians, nonprofessional hospital workers, and undoubtedly some others.

This paper surveys union wage effect estimates drawn from empirical cross-section wage equations fitted to macro (aggregated) data in 34 post-1963 studies containing such equations. Each estimate is the partial derivative of the dependent wage variable (in logarithmic units) with respect to the extent-of-unionism (fraction unionized) variable in the equation. This paper shows that these estimates contain a mixture in uncertain ratio of the union/nonunion wage differential or wage gap, on the one hand, and an "extent-of-unionism" effect, on the other. Therefore the Macro estimates should not be interpreted as wage gap estimates.