When the demand for a programme exceeds the supply, a lottery can be used to select the treatment group within the eligible population. Gertler states: When a programme is phased in, randomisation of the order in which participants receive the programme gives each eligible unit the same chance of receiving treatment in the first phase or in a later phase of the programme. As long as the last group has not yet been phased into the programme, it serves as a valid comparison group from which the counterfactual for the groups that have already been phased in can be estimated.