Evidence for sample selection effect and Hawthorne effect in behavioural HIV prevention trial among young women in a rural South African community

Authors

Molly Rosenberg, Indiana University
Audrey Pettifor, University of North Carolina
Rhian Twine, University of The Witwatersrand
James Hughes, University of Washington
F Gomez-Olive, University of The Witwatersrand
Ryan Wagner, University of The Witwatersrand
Afolabi Sulaimon, University of The Witwatersrand
Stephen Tollman, University of The Witwatersrand
Amanda Selin, University of North Carolina
Catherine L. Mac Phail, University of WollongongFollow
Kathleen Kahn, University of The Witwatersrand

RIS ID

118908

Publication Details

Rosenberg, M., Pettifor, A., Twine, R., Hughes, J. P., Gomez-Olive, F. X., Wagner, R. G., Sulaimon, A., Tollman, S., Selin, A., MacPhail, C. & Kahn, K. (2018). Evidence for sample selection effect and Hawthorne effect in behavioural HIV prevention trial among young women in a rural South African community. BMJ Open, 8 (1), e019167-1-e019167-9.

Abstract

Objectives We examined the potential influence of both sample selection effects and Hawthorne effects in the behavioural HIV Prevention Trial Network 068 study, designed to examine whether cash transfers conditional on school attendance reduce HIV acquisition in young South African women. We explored whether school enrolment among study participants differed from the underlying population, and whether differences existed at baseline (sample selection effect) or arose during study participation (Hawthorne effect). Methods We constructed a cohort of 3889 young women aged 11-20 years using data from the Agincourt Health and socio-Demographic Surveillance System. We compared school enrolment in 2011 (trial start) and 2015 (trial end) between those who did (n=1720) and did not (n=2169) enrol in the trial. To isolate the Hawthorne effect, we restricted the cohort to those enrolled in school in 2011. Results In 2011, trial participants were already more likely to be enrolled in school (99%) compared with non-participants (93%). However, this association was attenuated with covariate adjustment (adjusted risk difference (aRD) (95% CI): 2.9 (− 0.7 to 6.5)). Restricting to those enrolled in school in 2011, trial participants were also more likely to be enrolled in school in 2015 (aRD (95% CI): 4.9 (1.5 to 8.3)). The strength of associations increased with age. Conclusions Trial participants across both study arms were more likely to be enrolled in school than non-participants. Our findings suggest that both sample selection and Hawthorne effects may have diminished the differences in school enrolment between study arms, a plausible explanation for the null trial findings. The Hawthorne-specific findings generate hypotheses for how to structure school retention interventions to prevent HIV.