In sum, any IDA application must carefully consider both the mechanism that resulted in the sample of contributing studies and in the sample of individual observations within each study. Sampling simultaneously presents a danger and an opportunity within IDA. The danger is primarily a threat to internal validity such that effects perceived to be of theoretical importance may instead be attributable to heterogeneity in sampling mechanisms across the contributing studies. The opportunity is that sampling heterogeneity can become a topic of theoretical interest in and of itself such that gaining a better understanding of study-to-study differences can help us develop a better understanding of the substantive processes under study (e.g., Curran & Hussong, 2009). Regardless of intent, sampling is a critical issue that must be closely considered in any application of IDA.
