The key has been monitoring and evaluation that feed the strategy (Source "Poor Economics").
From their website:
We at Pratham believe that everything we do should be measurable and transparent. Monitoring and evaluation are integral aspects of all our programs. The reason is two fold:one, to ensure that we can measure the impact and two, to make changes in the strategy and model to increase efficacy based on the assessment.
Regular monitoring is essential to ensure that the programs are being carried out in an effective manner in the very last village/slum. Given the large expanse of our programs and the sheer number of people involved, a strong monitoring and evaluation system has been put in place.