essays in public finance and industrial organization a dissertation ...
essays in public finance and industrial organization a dissertation ...
essays in public finance and industrial organization a dissertation ...
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CHAPTER 3. MEDIGAP 136<br />
3.6.3 Robustness <strong>and</strong> External Validity<br />
Our first robustness check addresses the log specification for medical costs. It is well<br />
known that the skewed distribution of outcomes can create difficulties <strong>in</strong> estimat<strong>in</strong>g<br />
models with medical costs as a dependent variable. To show that the log specification<br />
we use is not driv<strong>in</strong>g the results, Panel A of Table 3.8 shows po<strong>in</strong>t estimates from the<br />
three-stage model where costs <strong>in</strong> the cost equation are denoted <strong>in</strong> levels. For com-<br />
parison, we divide these estimates by the average costs <strong>in</strong> our sample <strong>and</strong> display the<br />
results next to the estimated parameters from the model <strong>in</strong> logs. With demographic<br />
controls, the implied percentage changes <strong>in</strong> medical spend<strong>in</strong>g are similar to the log<br />
estimates (0.68 versus 0.57) although less precisely estimated.<br />
As discussed earlier, our analysis is restricted to the sample of <strong>in</strong>dividuals who<br />
turned 65 no sooner than the year 2000, as price data is not available from before<br />
this year. This, of course, means that the estimates should not be directly ascribed<br />
to the entire Medigap population because of potential age or cohort heterogeneity<br />
<strong>in</strong> the ma<strong>in</strong> effect. However, we can shed some light on the external validity of the<br />
ma<strong>in</strong> results with simple OLS regressions. Panel B of Table 3.8 shows OLS estimates<br />
of log medical costs on a Medigap <strong>in</strong>dicator for the entire sample of <strong>in</strong>dividuals <strong>in</strong><br />
the data <strong>and</strong> the subset we analyze. With demographic <strong>and</strong> health controls, the OLS<br />
estimates from the samples are very similar (0.34 versus 0.37). However, to the extent<br />
that health controls may be partly determ<strong>in</strong>ed by Medigap choice, the specification<br />
with only demographic covariates may be more appropriate. The lower OLS estimate<br />
for the full sample <strong>in</strong> this specification (0.11 versus 0.35) suggests some caution when<br />
extrapolat<strong>in</strong>g the estimates of out sample.<br />
F<strong>in</strong>ally, as discussed Section 3.5, there is <strong>in</strong> fact a small degree of with<strong>in</strong>-state<br />
variation <strong>in</strong> Medigap premiums <strong>in</strong> a small number of states <strong>in</strong> the sample (Maestas et<br />
al., 2009). To exam<strong>in</strong>e the robustness of the ma<strong>in</strong> results to this variation, we restrict<br />
our sample to states where, accord<strong>in</strong>g to Maestas et al., the with<strong>in</strong>-state coefficient of<br />
variation of Medigap premiums is less than 0.10 for 75 percent of firms issu<strong>in</strong>g plans. 20<br />
Panel C of Table 3.8 shows full model <strong>and</strong> OLS estimates of the coefficient on Medigap<br />
20 The states we drop are CA, FL, IL, LA, MI, NV, NY, PA, TX, <strong>and</strong> VA.