Key points are not available for this paper at this time.
Pradeep Kumar, Thanasis Stengos, Measuring the Union Relative Wage Impact: A Methodological Note, The Canadian Journal of Economics / Revue canadienne d'Economique, Vol. 18, No. 1, Econometrics Special (Feb., 1985), pp. 182-189
Kumar et al. (Fri,) studied this question.