The Catch-22 of External Validity in the Context of Constraints to Firm Growth
指出中小企业面临多种增长约束，导致商业培训项目效果难以评估，并探讨了外部有效性与样本选择之间的两难困境，强调需要更大样本、多臂评估和理论支持的实证研究。
We document the presence of multiple and varied constraints to small and medium firm growth. This presents both a practical problem for business training programs and a challenge to academic economists trying to identify mechanisms though which these programs may affect outcomes. External validity needs theory. This pushes researchers to narrowly defined and highly selected sample frames, which limits the potential for clear, generalizable policy prescriptions. Ultimately, larger samples, multi-arm evaluations, process documentation, and narrowly-focused, theory-supported empirical work are all needed, but the complexity of the problem limits what we learn from any single study.