This was a question posed by one of our readers in a comment on an earlier post I did on how to calculate the intra-class correlation in Stata.
cluster randomized trials
In clustered randomized experiments, random assignment occurs at the group level, with multiple units observed within each group. For example, education interventions might be assigned at the school level, with outcomes measured at the student level, or microfinance interventions might be assigned at the savings group level, with outcomes measured for individual clients.
While some of us get to conduct individually randomized trials, I’d say that cluster randomized trials are pretty much the norm in field experiments in economics. Add to that the increase in the level of ambition we recently acquired to have interventions with multiple treatment arms (rather than one treatment and one control group) and mix it with a pinch of logistical and budgetary constraints, we have a non-negligible number of trials with small numbers of clusters (schools, clinics, villages, etc.).