Peak HIV prevalence: a useful outcome variable for ecological studies.

A key question for ecological studies with HIV as the outcome variable is what measure of HIV prevalence to use. In this study we compared the strengths and weaknesses of a variety of measures of HIV prevalence, focusing on peak HIV prevalence and HIV prevalence measured at the same time as the exposure variable. We explored the theoretical problems with each of the two measures of HIV prevalence. We then investigated the difference that substituting one variable for the other made to two published ecological studies. One published study found a strong relationship between migration intensity and HIV prevalence measured at the time the migration was measured. When we repeated the analysis using peak HIV prevalence as the outcome variable, there was no evidence of an association. The second study found evidence of a strong relationship between concurrency and peak HIV prevalence. On repetition of the analysis (but utilizing HIV prevalence at the time the concurrency was measured as the outcome variable) there was no longer a significant association. The choice of HIV measure as outcome variable in ecological studies makes a large difference to the study results. The choice of peak HIV prevalence as outcome variable offers the advantage of avoiding the HIV introduction time bias.