Evidence of high bednet usage from a list randomization experiment in rural Gambia

Background Recording behaviours that have the potential to impact health can be doubly challenging if the behaviour takes place in private spaces that cannot be observed directly and where there is potential for social desirability bias, i.e. where the respondent may give an answer that they think the recorder wants to hear. Sleeping under a long-lasting insecticidal net (LLIN) is an important intervention for malaria prevention, yet it is difficult to gauge the extent to which coverage (how many nets are in the community) differs from usage (how many people sleep under a net). We employed a novel method, list randomization, which partially obscures respondents' answers to sensitive questions, as a mean to provide an accurate estimate of LLIN usage in The Gambia.Methods We surveyed 196 residents from 196 households recruited into a randomised controlled trial assessing the effect of a housing intervention on malaria. In the current experiment, 98 of the 196 study participants were randomly assigned to the control group and received a four-question list about non-sensitive behaviours; the other participants in the intervention group, received the same list, with the addition of one question on a sensitive behaviour; whether or not they had used a bednet the previous night. Participants were read the list of questions and then said how many of the statements were true. We estimated bednet usage by calculating the difference in means between the total number of affirmative items between the two groups, and quantified uncertainty using a t-test.Results The mean number of affirmative responses in the control group was 2.60 of four statements (95% confidence interval, 95% CI, = 2.50-2.70), compared with 3.68 (95% CI = 3.59-3.78) in the intervention group. Such difference (1.08; 95% CI = 94.9-100%) suggests approximately 100% bednet usage.Conclusions Our findings suggest complete universal bednet usage in the study area. Further validation of the list randomization method in areas with lower net coverage is required.