A comment on Siegfried's first lesson in econometrics

In an insightful, and delightfully short, paper that was published in the Journal of Political Economy in 1970, Siegfried provided an im- portant lesson about econometric methodology for all budding young econometricians. Unfortunately, there is a mistake in that paper. This short comment provides a simple correction for that mistake. This correction will hopefully enable econometrics students to obtain the fundamental insights from Siegfried's paper with less confusion than might otherwise have been the case.