Abhijeet has a new post up at the CSAE blog.
He makes the critical point that when a study finds no difference between regular teachers and contract teachers in Kenya, those contract teachers are still being paid only a fifth of regular teachers. It's kind of silly to be disappointed by seeing no positive impact (but no negative impact either) when you are paying a fifth of the cost. (edit: the above isn't actually correct - see the discussion in the comments here - the quote below is still accurate however)
across all these studies, contract teachers never do worse than civil service teachers, despite being younger, more inexperienced and more likely to not have had formal teacher training. In value-for-money terms, each contract teacher is at least four to five times as productive as a regular public service teacher: in Kenya, the average pay of a civil service teacher is $261 per month compared to $56 for a contract teacher. The problem is not that the contract teachers are being paid too little (they get paid salaries comparable to private school teachers in these countries) but that public sector employment just has a huge premium attached.