After explaining that the Maryland Scientific Methods Scale (SMS) is largely based on a classic book by Cook and Campbell, the authors describe the testing of causal hypotheses by focusing on the issues of statistical conclusions and internal, construct, and external validity. Arguing that there have been many previous studies that have attempted to devise scales of methodological quality, the authors assert that the SMS scale was devised as a simple scale to measure internal validity. Stating that the SMS scale easily communicates research findings to scholars, policymakers, and practitioners, the authors assess the strengths and weaknesses of the SMS, arguing that one problem with the scale is that it is designed to apply equally to all experimental units. Contending that the 5-point SMS scale of internal validity might be improved by researchers creating larger units of analysis and recognizing links in the causal chain between effects of such variables as X and Y, the authors caution that the very changes that might improve the quality of the results generated by the SMS may also compromise the simplicity of this scale. References