Re-analysis of health and educational impacts of a school-based deworming programme in western Kenya: a statistical replication of a cluster quasi-randomized stepped-wedge trial

Abstract

Introduction: Helminth (worm) infections cause morbidity among poor communities worldwide. An influential study conducted in Kenya in 1998-99 reported that a school-based drug-and-educational intervention had benefits for worm infections and school attendance.

Methods: In this statistical replication, we re-analysed data from this cluster quasi-randomized stepped-wedge trial, specifying two co-primary outcomes: school attendance and examination performance. We estimated intention-to-treat effects using year-stratified cluster-summary analysis and observation-level random-effects regression, and combined both years with a random-effects model accounting for year. The participants were not blinded to allocation status, and other interventions were concurrently conducted in a sub-set of schools. A protocol guiding outcome data collection was not available.

Results: Quasi-randomization resulted in three similar groups of 25 schools. There was a substantial amount of missing data. In year-stratified cluster-summary analysis, there was no clear evidence for improvement in either school attendance or examination performance. In year-stratified regression models, there was some evidence of improvement in school attendance [adjusted odds ratios (aOR): year 1: 1.48, 95% confidence interval (CI) 0.88-2.52, P = 0.147; year 2: 1.23, 95% CI 1.01-1.51, P = 0.044], but not examination performance (adjusted differences: year 1: -0.135, 95% CI -0.323-0.054, P = 0.161; year 2: -0.017, 95% CI -0.201-0.166, P = 0.854). When both years were combined, there was strong evidence of an effect on attendance (aOR 1.82, 95% CI 1.74-1.91, P < 0.001), but not examination performance (adjusted difference -0.121, 95% CI -0.293-0.052, P = 0.169).

Conclusions: The evidence supporting an improvement in school attendance differed by analysis method. This, and various other important limitations of the data, caution against over-interpretation of the results. We find that the study provides some evidence, but with high risk of bias, that a school-based drug-treatment and health-education intervention improved school attendance and no evidence of effect on examination performance.

Keywords: Helminth; Kenya; primary schools; randomized control trial; worms parasitic.

Figure 1.

CONSORT diagram. Study participants were enumerated at the start of the study if they were registered at school in grades 1–8 at the start of 1998. Follow-up is shown for primary outcome data. ‘Pupil-observations’ refers to the number of times that any pupils were observed in the group, excluding observations after transferring schools. The levels of missingness for pupil-observations is calculated for visits that took place, and therefore does not incorporate missingness due to schools not being visited. Missingness for examination data is based on the number of pupils in standards 3–8 who had not moved school. Socioecon., socioeconomic status; NGO, non-governmental organization.

Figure 2.

Stepped-wedge design. Stepped-wedge design shown in schematic form. The intervention was rolled out in ‘steps’, with Group 1 receiving the intervention in year 1, Group 2 in year 2 and Group 3 in the year after the study.

Figure 3.

Scatter-plot of proportion present against number of observations, by year and group. Scatter plots of proportion present against the number of observations in each school by year and by allocation group. The dotted line indicates 95% attendance. Two schools in Group 2 have been excluded from the charts to preserve the scale: one in year 2 (1999) where no pupils are recorded present, and one in year 1 (1998) with a disproportionately large number of observations (approximately 6000).