Final answer:
Smith, Glass, and Miller, using meta-analysis in 1980, found that psychotherapy is generally effective, with various approaches offering similar levels of success, challenging the idea of significant differences in psychotherapy outcomes.
Step-by-step explanation:
Smith, Glass, and Miller conducted a groundbreaking study in 1980 where they utilized meta-analysis to assess the efficacy of psychotherapies. They reviewed hundreds of studies to determine how effective different types of psychotherapy were in treating a range of psychological conditions.
Their work demonstrated that, on average, psychotherapy had a positive effect on clients, with various approaches yielding similar levels of effectiveness.
This finding challenged the prevailing notion that certain psychotherapies had significantly different outcomes from others and established meta-analysis as a valuable tool in the field of psychology research.