The Poisson Approximation to the Binomial Distribution

Econometrics textbooks make use of the assumption of a fixed regressor matrix, although it is nearly always unrealistic for economic data. For developing the properties of the general linear model, this is a convenient simplification. But this assumption is retained when examining model specification. This practice has generated results that, if not erroneous, are certainly incomplete and thus deceptive. In particular, the omission of a variable or the imposition of any incorrect restriction may well increase the variance of estimated parameters rather than reduce it, as stated in the textbooks.