Population-level analyses of SMOs typically have relied on a single source for data, most commonly the Encyclopedia of Associations (EoA). However, the validity of this procedure has been drawn into question by recent organizational studies. To examine the impact of using different sources to estimate SMO populations, we compile a comprehensive population dataset of national and regional U.S. environmental movement organizations (or EMOs) over a 100-year time period using 155 different sources. We use this data to evaluate the accuracy and selection biases in five major compilations of U.S. EMOs. The analysis shows that all single sources are selective, tapping specific sections of the environmental movement. Multiple sources are needed to capture a comprehensive population of EMOs. Researchers should be aware of the limitations of specific sources before drawing conclusions about population parameters.
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