The Econometric SocietyEdited by: Guido W. Imbens • Print ISSN: 0012-9682 • Online ISSN: 1468-0262
August 15, 2022
Econometrica: May, 1971, Volume 39, Issue 3
https://doi.org/0012-9682(197105)39:3<461:EASEIA>2.0.CO;2-0
p. 461-465
Walter D. Fisher, Walter J. Wadycki
In two stage least squares estimation, when the matrix Z of predetermined variables has less than full column rank, Z'Z is singular. It is shown that in this situation 2SLS still can be used and that Y - the result of the first stage--is still uniquely determined; also shown is that 2SLS will often be equivalent to OLS.
