mortality even when possible selection bias is taken<sup>into</sup> account.

In both of the other African studies by Aaby, et<sup>al. (1984)</sup> and Garenne and Caitrelle (1986), control groups<sup>consisted of</sup> unvaccinated children from the same areas where measles vaccination was offered. As such, selection bias<sup>may represent an</sup> important confounding factor, with controls disproportionately comprised of children who face independently higher mortality risks. This may account in part for the extremely<sup>large</sup> reductions in mortality reported by the Aaby, et al.<sup>in the</sup> Guinea-Bissau study.

In the case-control study by Clemens, et al. (1988)<sup>in</sup> Matlab, little or no effect of measles vaccination upon mortality would have been expected during the first<sup>year of the</sup> study, since the program was introduced at the end of the annual measles season, unless vaccination acceptors constituted<sup>a</sup> special group with lower overall mortality risks, The<sup>finding</sup> that there was no protective effect from measles vaccination during the first year of the study provides strong evidence<sup>that</sup> measles vaccinees did not constitute a select group with<sup>better</sup> survival chances from the onset (WHO, 1987). In addition,<sup>the</sup> reported finding by the authors of a 36 percent mortality<sup>risk</sup> among vaccinated children was based upon multivariate<sup>analysis</sup> which controlled for the effects of a number of<sup>potentially</sup> confounding demographic and socioeconomic characteristics<sup>which.</sup> might differentiate vaccinated from unvaccinated children.

The Matlab cohort study by Koenig, et al. (1990)<sup>also</sup> considered the issue of potential selection bias from several different perspectives. First, the study examined whether<sup>the</sup> families of vaccinated and unvaccinated children<sup>differed on</sup> other indicators of health behavior which might account for lower mortality among the former group of children. No difference between groups was evident with respect to one important indicator of health behavior-- use of contraception.<sup>Second, the</sup> authors carried out multivariate analysis to control for<sup>the</sup> effects of a number of potentially confounding variables<sup>(birth</sup> order, sex, maternal education, and area of household)<sup>on which</sup> vaccinated and unvaccinated children might differ. The<sup>results</sup> indicated that even after controlling for these factors, vaccinated children still experienced mortality levels<sup>which were</sup> 46 percent lower. Lastly, the authors examined whether vaccinated children from the two blocks where measles vaccine<sup>was offered</sup> may have been self-selected for better survival-- i.e., that a primary reason why children remained unvaccinated<sup>in these blocks</sup> was that they failed to live long enough to be vaccinated. To take this into account, they constructed a second<sup>data set which</sup> included all children-- both vaccinated as well as unvaccinated-from the two blocks where measles vaccination<sup>had been offered,</sup> and randomly rematched with the pool of unvaccinated<sup>children</sup> 15