Efficient Estimation: The Rao‐Zyskind Condition, Kruskal's Theorem and Ordinary Least Squares*

The paper emphasizes the practicability and accessibility of the necessary and sufficient condition for ordinary least squares to yield best linear unbiased estimators in several problems that are available in econometrics Two convenient equivalent alternative forms of the condition are presented It is shown that the condition is useful for analyzing different problems and is especially relevant for pedagogical purposes Several practical economic examples are presented