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Thursday, October 25, 2007

Non RANDom panel attrition update II (briefer version of update I)

I enthuse about a criticism by John Nyman of a RAND study on the effect of copayments on demand for health care and health outcomes.

Still following in the footsteps of a kid half my age, I note that Ezra Klein notes that RAND has responded to the criticism with this pdf.

The RAND study was a genuine experiment in which participants were randomly assigned to complete coverage or one of 6 different plans with copayments. RAND concluded that demand for health care was reduced by copayments but found significant worsening of health outcomes only for some poor participants.

The original Nyman critic noted that participants with less generous programs were much more likely to switch back to their original insurance plans (as they were allowed to do). If sicker people did so, this could bias the result.

RAND attempted to follow the health outcomes of people who switched out of the experimental plans and has data on health outcomes of 77% of them. Thus the attrition problem is much smaller than I thought after reading Nyman's abstract (I would have had to pay to read his actual article).

I am very struck by the following paragraph in the reply

4. Moreover, Nyman’s speculation about a high degree of non-random attrition
is contradicted in work that is unpublished but also posted on Newhouse’s
home page. In this work Manning, Duan, and Keeler carried out additional
analyses of those who did not complete the Experiment. They concluded that
there was in fact a modest amount of non-random attrition, but that its effects,
if accounted for, would have left our conclusion that cost sharing reduced use

This is striking because there is no claim that accounting for non-random attrition would leave the conclusion that the health effects of reduced use of care due to copayments were negligible.

The web page is here

the *.pdf by Manning Duan and Keeler is the third article from the top on the web page.

I quote from Manning Duan and Keeler

"Dropouts were sicker on average at enrollment than those who stayed,"

The last statement is very very surprising in an article cited in a reply to the assertion that the vastly greater number of dropouts from the plans with copays biased up the average health status of people assigned to those plans.

In fact, the reply seems to be based on the argument that 77% = 100% which is not exactly true is it ?

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