In 1998, a local non-governmental organization (NGO) launched a program known as the Primary School Deworming Program (PSDP) to provide deworming medication to individuals enrolled in 75 primary schools in Busia District, a densely-settled farming region of rural western Kenya adjacent to Lake Victoria. The schools participating in the program consisted of nearly all rural primary schools in Budalangi and Funyula divisions in southern Busia district, and contained more than 30,000 pupils at the start of the study. Baseline parasitological surveys conducted by the Kenyan Ministry of Health indicated that these divisions had high rates of helminth infection at over 90%. Using modified WHO infection thresholds (Brooker et al., 2000b), roughly one-third of children in the sample had “moderate to heavy” infections with at least one helminth at the time of the baseline survey, a rate not atypical by regional standards (Brooker et al., 2000a). The 1998 Kenya DHS indicated that 85% of children in western Kenya, in the relevant age range of 8-18 years, were enrolled in school – suggesting that the sample was broadly representative of western Kenyan children as a whole at the time.
The 75 program schools were randomly divided into three groups (Groups 1, 2, and 3) of 25 schools each: the schools were stratified by geographical area (division, then zone), the zones were listed alphabetically (within each division), and then within each zone the schools were listed in increasing order of student enrolment, and every third school was assigned to a given project group. Due to the NGO’s administrative and financial constraints, the schools were phased into the program over the course of 1998-2001, and the order of phase-in was randomly determined, creating experimental treatment groups. Group 1 schools began receiving free deworming in 1998, Group 2 schools in 1999, while Group 3 schools began receiving the drugs in 2001. The project design implies that in 1998, Group 1 schools were treatment schools while Group 2 and 3 schools were the control, and in 1999 and 2000, Group 1 and 2 schools were the treatment schools and Group 3 schools the control, and so on. In 2002 all schools received free treatment. Children in Group 1 and 2 schools thus received two to three more years of deworming than Group 3 children. Deworming drugs were offered twice per year in treatment schools.