Final Report on the Effects of Sample Attrition on Estimates of Channeling's Impacts. III. HOW ATTRITION CAN LEAD TO BIAS AND A STATISTICAL PROCEDURE FOR ELIMINATING THE BIAS


In the previous chapter we found that attrition produced no systematic pattern of treatment/control differences in the observed, initial characteristics of the samples available for analysis. This finding does not rule out the possibility that at the time of follow up the two groups differ on unmeasured characteristics that also affect the level of a particular outcome measure. If, as a consequence of attrition, the treatment and control groups differ on average on these unmeasured characteristics, then impact estimates that do not control for these differences will be biased.

In this chapter we outline, first informally and later using statistical notation, the conditions under which attrition bias may arise. Next, we outline a statistical procedure, due to Heckman (1976, 1979) that corrects for possible bias. Finally, we show how the direction of the bias can be determined when some prior knowledge exists concerning the mechanism causing attrition.

We focus here on giving a heuristic explanation of the procedures. More complete coverage of statistical details and derivations of these methods and their justification is given in the references listed at the end of this report.

View full report


"atritn.pdf" (pdf, 4.74Mb)

Note: Documents in PDF format require the Adobe Acrobat Reader®. If you experience problems with PDF documents, please download the latest version of the Reader®