Selection Bias Adjustment in Treatment-Effect Models as a Method of Aggregation

The aim of this note is to interpret estimation of the conventional treatment-effect selection-bias model in econometrics as a method of aggregation and to draw the implications of this interpretation. In addition, the paper notes the connection of this interpretation with an older style of analysis using grouped data and illustrates the aggregation analogy with examples from the literature. The estimation technique used to illustrate the points is the method of instrumental variables.