9. SENSITIVITY ANALYSIS
If the purpose of my essay had been a comprehensive review of
econometric evaluation estimators, I would have discussed bounds
and sensitivity analysis. See Heckman and Vytlacil (2006b) for such a
discussion using the MTE as an organizing device for an entire
literature. Vijverberg (1993) is a good reference for sensitivity analysis
in a Roy model. Peterson (1976) is an early example of bounds for the
competing risks model which is a version of the Roy model.
28
10. POLICY FORECASTING
Sobel explores a few special cases of my analysis of problem P2. This
section of his discussion abounds with personal opinions like ‘‘intui-
tively easy’’ and ‘‘think reasonably.’’ These assertions are his excuse
for begging the general problems considered in my paper. If he does
not like unobservables, he can use standard change of variable argu-
ments to substitute out unobservables from my equations and recast
my argument into observables to solve identification problems.
My argument uses structural models directly because they are
interpretable in terms of theory and they explicitly recognize missing
variables (‘‘unobservables’’). They also provide the machinery for
integrating information on auxiliary measures into analyses to help
overcome problems with missing variables.
His analysis makes implicit exogeneity assumptions. See
Heckman and Vytlacil (2005, 2006b) for a general analysis of the
extrapolation problem based on the MTE which explicitly discusses
the role of exogeneity assumptions in policy forecasts.
11. THE ROLE OF UNOBSERVABLES
Sobel, like many statisticians, says he does not like unobservables. Neither
do I. I wish all important variables were observed. But it is the unobser-
vables in the outcome equations and the outcomes in the treatment choice
28
See Heckman and Honore´ (1989, 1990).
156
HECKMAN
equation that give rise to selection and evaluation problems. One can be
implicit about them and their properties, as are Sobel and most statisti-
cians, or explicit, as is common in the econometric approach.
Implicit in all statistical analyses are unobservables that gen-
erate outcomes given the observables. Matching assumes that all
unobservables in outcome equations are random with respect to the
unobservables in the treatment choice equation given the matching
variables. The LATE estimator that Sobel implicitly espouses assumes
a latent unobserved variable with a special structure.
29
By being clear
and objective, the econometric approach allows analysts to pinpoint
their differences in assumptions about the unobservables and explore
the role of assumptions in producing any differences in conclusions.
12. TOWARDS CONVERGENCE
I applaud Sobel’s desire to see the emergence of a literature that
combines the best features of the econometric and statistical treatment
effect literatures. For that synthesis to occur, statisticians like Sobel
should carefully read the econometrics literature and its genuine pio-
neering contributions and more carefully note the publication dates of
key ideas before dismissing 30 years of econometric research as the
application of an incompletely formulated statistical model.
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