Smith and Glass's Psychotherapy Outcome Meta-Analysis

In 1977 the researchers Mary Lee Smith and Gene Glass published a landmark study in American Psychologist that statistically combined the results of three hundred seventy-five separate controlled studies of psychotherapy and counseling outcomes, gathered from a search of roughly a thousand journals, magazines and books, using a then-novel statistical technique that Glass had recently named meta-analysis. Rather than simply counting how many individual studies found statistically significant benefits, as earlier narrative reviews of the psychotherapy outcome literature had done, Smith and Glass calculated a standardized measure of effect size across the pooled studies, allowing them to estimate the overall average benefit of psychotherapy in a single combined statistic. Their analysis found that the average person who received psychotherapy or counseling ended up better off than roughly seventy-five percent of people in the untreated comparison groups, providing quantitative support for the general effectiveness of psychotherapy at a time when its value had been publicly challenged by earlier critics. The analysis also found relatively few consistent differences in effectiveness among the many different types of therapy compared, including behavioral and non-behavioral approaches, a pattern that later became known in the field as the common factors debate over why different therapies appear to produce broadly similar outcomes. Beyond its specific findings, the study is widely credited with establishing meta-analysis itself as a standard statistical method across psychology and the social sciences more broadly.