Please use this identifier to cite or link to this item: https://hdl.handle.net/10419/276270 
Year of Publication: 
2023
Series/Report no.: 
Working Paper No. 426
Version Description: 
Revised version, May 2023
Publisher: 
University of Zurich, Department of Economics, Zurich
Abstract: 
This paper explores an algebraic relationship between two types of coefficients for a regression with several predictors and a group structure. In a general regression, the regression coefficients are allowed to be group-specific, the restricted regression imposes constant coefficients. The key result is that the restricted coefficients are not necessarily a convex average of the group-specific coefficients. In the context of regression with two independent variables and two groups, I show that the coefficient of a regressor estimated from pooled data can be negative, even though the separately estimated coefficients are positive in each group, providing an additional example of Simpson's paradox.
Subjects: 
Covariance-weighting
non-convex average
average treatment effect
JEL: 
C21
Persistent Identifier of the first edition: 
Document Type: 
Working Paper

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