"The trade-offs between qualitative and quantitative research methods are well known. Qualitative research promises high internal validity and the ability to disentangle causal processes. Given the costs of conducting in-depth research, however, it is difficult to obtain
the large number of qualitative observations required to establish external validity. We elaborate upon these challenges and discuss the relative merits and shortcomings of three strategies for building large-N databases using qualitative research: large-N field-based
studies conducted by one or a few researchers, meta-databases constructed from existing qualitative studies, and large-N field-based studies conducted by research networks. Examples are drawn from research on collective action to manage natural resources."