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differential attrition

Attrition rates typically aren’t that different for the control group than the treatment group – really? and why?

David McKenzie's picture

When I start discussing evaluations with government partners, and note the need for us to follow and survey over time a control group who did not get the program, one of the first questions I always get is “Won’t it be really hard to get them to respond?”. I often answer with reference to a couple of case examples from my own work, but now have a new answer courtesy of a new paper on testing for attrition bias in experiments by Dalia Ghanem, Sarojini Hirshleifer and Karen Ortiz-Becerra.

As part of the paper, they conduct a systematic review of field experiments with baseline data published in the top 5 economics journals plus the AEJ Applied, EJ, ReStat, and JDE over the years 2009 to 2015”, covering 84 journal articles. They note that attrition is a common problem, with 43% of these experiments having attrition rates over 15% and 68% having attrition rates over 5%. The paper then has discussion over what the appropriate tests should be to figure out whether this is a problem. But I wanted to highlight this panel from Figure 1 in their paper, which plots the absolute value of the difference in attrition rates by treatment and control. They note “64% have a differential rate that is less than 2 percentage points, and only 10% have a differential attrition rate that is greater than 5 percentage points.” That is, attrition rates aren’t much different for the control group.

Sometimes (increasingly often times), estimating only the ITT is not enough in a RCT

Berk Ozler's picture

"In summary, the similarities between follow-up studies with and without baseline randomization are becoming increasingly apparent as more randomized trials study the effects of sustained interventions over long periods in real world settings. What started as a randomized trial may effectively become an observational study that requires analyses that complement, but go beyond, intention-to-treat analyses. A key obstacle in the adoption of these complementary methods is a widespread reluctance to accept that overcoming the limitations of intention-to-treat analyses necessitates untestable assumptions. Embracing these more sophisticated analyses will require a new framework for both the design and conduct of randomized trials."