The PSD Blog, citing Chris Blattman, wonders if randomized evaluation is too expensive:

In some circumstances, simple monitoring and evaluation without randomization might be enough, even if it leaves us with less certainty about the outcomes. Perhaps it’s time to develop more formal cost-benefit guidelines for randomized evaluations?

Our problem is that we are investing too little in rigorous evaluations, not too much.  We’ve been giving aid for 50 years now with pitifully little evidence about what really works. 

Of course it is true in principle that we could invest too much in evaluation – but the point of diminishing returns is a long way above where we are now.  The sums of money involved are trivial by comparison with the huge amounts of aid we are spending on the basis of far too little information.

I’m reminded of Derek Curtis Bok’s famous remark:

If you think education is expensive, try ignorance.