Individual randomisations are relatively low-cost as sample size requirements are lower. However, such a randomisation within a village faces threats to internal validity. The first risk, namely that of contamination by having individuals ‘switching’ their treatment status, was addressed by making the insurance/credit contracts non-transferable. The other main threat is known as the John Henry effect, when the control group changes its behaviour due to knowledge of what is happening in the treatment group, and in this case, for example, is triggered to seek traditional credit from other banks operating in the region. While the research team may not be able to control this, by collecting information on the credit and source of loan, they will be able to identify and assess the magnitude of this problem.

The second type of example is of cluster-based randomisation, a cluster being a grouping of individuals or households at a level which makes sense from a point of view of the intervention and outcome of interest (e.g.