Final Report on the Effects of Sample Attrition on Estimates of Channeling's Impacts. C. Summary


The comparison of full and partial analysis samples indicates that estimates of hospital impacts are not biased by restricting that analysis to the Medicare sample. For nursing home impacts, the evidence suggests that there is probably no bias, although this inference is based on estimated impacts on only a small portion of total nursing home use and other outcome variables that may be correlated with nursing home use. For well-being outcomes, the comparison of impacts on Medicare-covered services estimated on the full and followup samples provides indirect evidence of the absence of bias: the two sets of estimates are quite similar, suggesting that the sample available for analysis is not very different from the full sample. Finally, for impacts on formal and informal care the comparison of full and in-community sample estimates did reveal a difference in the statistical significance of the estimates for two service use outcome measures, but only for one model, in one time period. The lack of consistent differences across outcome measures, time periods, and models, together with the fact that the observed differences were for the model with the smallest treatment/control difference in attrition rates suggests that estimates of channeling impacts on formal and informal care outcomes are not biased by attrition either.

The evidence in this chapter thus strongly suggests that attrition did not lead to biases in any area of analysis. However, since the evidence is indirect for outcomes other than hospital use, we employ the statistical procedure described in Chapter III to obtain further evidence on whether attrition bias exists for the nursing home, followup, and in-community samples.

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