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Do response propensities change with repeated calling?

I read a very interesting article by Mike Brick. The discussion of changing propensities in section 7 on pages 341-342 was particularly interesting. He discusses the interpretation of changes in average estimated response propensities over time. Is it due to changes in the composition of the active sample? Or, is it due to within-unit decreases in probability caused by repeated application of the same protocol (i.e. more calls)?

To me, it seems evident that people's propensity to respond do change. We can increase a person's probability of response by offering an incentive. We can decrease another person's probability by saying "the wrong thing" during the survey introduction.

But the article specifically discusses whether additional calls actually change the callee's probability of response. In most models, the number of calls is a very powerful predictor. Each additional call lowers the probability of response. Brick points out that there are two interpretations of that. Either each call reduces the probability for each case, or as the mixture of active cases shifts toward a larger proportion of more difficult cases the average probability declines. 

In this case, I thought the latter explanation was more likely. In fact, a paper I wrote on estimating contact probabilities at the household-level makes the assumption (which is also sometimes wrong) that the household probability of contact is fixed within any window and can be more precisely estimated with repeated trials. I explicitly argued that the average "8th call" probability of contact was not useful for planning a strategy for calling any household as it is simply the average contact probability for a set of difficult to contact cases.

I thought the article did a good job of outlining this controversy in a very clear way.

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