This video lecture given by Dr Annette Brown for the Asian Development Bank (ADB) and the International Initiative for Impact Evaluation (3ie) describes how to create a valid counterfactual using randomized control trials (RCT). Using easy to follow language, graphics and specific examples she goes on to demonstrate how random assignment works in practice.
This is lecture three in a six part series that introduce impact evaluations. The other video lectures are as follows:
Brown, A. (2014). Introduction to Randomized Control Trials, Asian Development Bank (ADB) and the International Initiative for Impact Evaluation (3ie). Retrieved from: http://impactevaluation2014.org/introduction-to-randomized-control-trials/