From the Guardian, 2012
The hypothesis In the 1990s, a Dutch development charity called International Christelijk Steunfonds decided to fund a programme to support education in Kenya. Previous research had suggested that providing African children with textbooks that they could not normally afford might improve their exam results, so the charity paid for 25 schools to receive sets of English, science and maths books. The charity, however, didn't just provide the books. It decided to run an experiment.
The experiment As Tim Harford describes in his book Adapt, ICS asked the Kenyan government not to select 25 schools that would receive the books, but to identify 100 schools that would be equally suitable. From these, 25 were selected at random. The books were delivered and exam results at the 25 intervention schools compared with those from the 75 similar schools without the extra teaching resources.
The textbooks, it turned out, made very little difference. ICS then tried another intervention – illustrated teaching flip-charts – in a similar randomised trial. Again, there was no significant effect.
=So the charity tried a third approach, funding treatment for intestinal worms. This time, the trial followed a staggered design: 25 random schools received the treatment immediately, 25 after two years, and another 25 two years after that. This time, there was clear evidence: de-worming children unequivocally improved their learning, probably thanks to improved nutrition.