How do you allocate not too much, not too little, but just the right amount of resource to measuring a programme's effectiveness? Spend too little time tracking the efficacy of development programmes and you risk wasting resources, not to mention losing the confidence of your backers. But spend too much time and you could still end up wasting resources, thus doing less of the good you initially set out to do. Christian Jahn and Susann Tischendorf on how to approach the 'Goldilocks dilemma' of tracking impact.