Cross section estimates of the union wage effect are typically much
larger than estimates derived from within estimators using panel data. Two
competing explanations for this difference have been advanced. The first is
that the cross section estimates suffer from an omitted variables bias due to
a correlation between unobserved productivity and union status which biases
the cross section estimator upwards. The second is that measurement error
in union status is more severe in the changes than in the levels, imparting a
more severe downward bias to the panel estimator.
This paper derives a method of moments estimator which allows for
both effects, nested within the same model. The binary nature of the
explanatory variable is exploited to derive an estimating model which allows
simultaneous estimation of both the structural parameters of the model and
the parameters of the measurement error process. When the estimator is
applied to sample of men from the PSID we find that allowing for measurement
error does lead to a larger estimate of the union wage effect than the usual
within estimator, but that most of the difference between the cross section
and the panel estimates is not due to measurement error in the union
variable. Further, the estimates of the extent of measurement error are
close to those found in a validation study of the PSID.