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CHAPTER 3. MEDIGAP 132<br />

policy that could not plausibly be related to the unobserved component <strong>in</strong> the cost<br />

equation. MA penetration decreased over the first part of our sample as policies<br />

enacted <strong>in</strong> the 1997 Balanced Budget Act to reduce Medicare payments <strong>in</strong>directly<br />

decreased MA payments. MA penetration rates were then buoyed up by <strong>in</strong>creased<br />

payments written <strong>in</strong>to the 2003 Medicare Modernization Act (?). In addition, the<br />

goal of MA has shifted over time from promot<strong>in</strong>g efficiency to <strong>in</strong>clud<strong>in</strong>g broader goals<br />

such as regional equity mean<strong>in</strong>g there has been great variation over time <strong>in</strong> federal<br />

policy toward regional MA <strong>in</strong>surers. Thus most of the changes over time were driven<br />

by federal policy that could not be related to the unobservable.<br />

We also statistically test for the endogeneity of the county-year specific MA pen-<br />

etration variables. Recall that our Meidgap choice equation has two sets of excluded<br />

variables: Medigap premiums <strong>and</strong> the MA penetration variables. Based on the as-<br />

sumption that estimates from a specification us<strong>in</strong>g only Medigap premiums are con-<br />

sistent but <strong>in</strong>efficient <strong>and</strong> estimates from a specification with both Medigap premiums<br />

<strong>and</strong> MA penetration are efficient, a Durb<strong>in</strong>-Wu-Hausman test squarely fails to reject<br />

the exogeneity of MA penetration variables. In our preferred specification with the<br />

full set of controls, the test returns a Chi-squared value of 0.8, a value vastly below<br />

the 5 percent critical value of 37.6. Tests run on other specifications yield similar<br />

results.<br />

3.6 Results <strong>and</strong> Discussion<br />

3.6.1 Premiums <strong>and</strong> Medigap Choice<br />

Below we present basel<strong>in</strong>e estimates of the empirical model, start<strong>in</strong>g with the first<br />

stage premium equation, followed by the mult<strong>in</strong>omial logit second stage Medigap<br />

choice equations, <strong>and</strong> f<strong>in</strong>ally the third stage cost equation. Each of these equations is<br />

estimated with three different sets of control variables. A sparse specification <strong>in</strong>clud-<br />

<strong>in</strong>g only key variables <strong>and</strong> year fixed effects; a specification that <strong>in</strong>cludes the demo-<br />

graphic controls; <strong>and</strong> a full specification that, along with the demographic variables,

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