Jacob Cohen (scientist)

Jacob Cohen (1923 – 2014) was an American statistician and psychologist best known for his work on statistical power analysis and effect size. He is widely regarded as a pioneer in promoting the understanding and use of these concepts in behavioral and social sciences research. Cohen was a distinguished professor of psychology at New York University.

Contributions:

• Statistical Power Analysis: Cohen's most significant contribution was to popularize statistical power analysis. His book, Statistical Power Analysis for the Behavioral Sciences (first published in 1969), became a standard reference for researchers planning studies. He provided accessible tables and guidelines to help researchers determine the sample size needed to achieve a desired level of statistical power, which is the probability of finding a statistically significant effect if one truly exists. He emphasized the importance of power analysis in study design to avoid type II errors (false negatives).

• Effect Size Measures: Cohen advocated for the routine reporting and interpretation of effect sizes alongside statistical significance tests. He developed and promoted several effect size measures, including Cohen's d (for measuring the standardized difference between two means) and Cohen's f² (for measuring the effect size in ANOVA). He provided guidelines for interpreting the magnitude of these effect sizes, defining conventions such as small, medium, and large effects, which although sometimes criticized as overly rigid, offered a helpful framework for understanding the practical significance of research findings.

• Beyond Hypothesis Testing: Cohen was a strong proponent of moving beyond the sole reliance on null hypothesis significance testing (NHST). He argued that NHST, while useful, provides limited information about the magnitude and importance of an effect. He encouraged researchers to focus on estimating effect sizes and interpreting them in the context of the research question.

• Methodological Reform: Cohen's work played a significant role in the broader movement toward methodological reform in the behavioral and social sciences. His emphasis on power analysis, effect sizes, and moving beyond NHST has contributed to more rigorous and informative research practices.

Selected Publications:

• Cohen, J. (1969). Statistical power analysis for the behavioral sciences. New York: Academic Press. (Later editions were published in 1977 and 1988)
• Cohen, J. (1994). The earth is round (p < .05). American Psychologist, 49(12), 997-1003.
• Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ: Lawrence Erlbaum Associates.

Legacy:

Jacob Cohen's work continues to have a profound influence on research practices in psychology, education, and other fields. His contributions have helped researchers to design more powerful studies, interpret results more meaningfully, and move beyond the limitations of traditional statistical methods. His writings remain highly cited and his ideas are widely taught in statistics and research methods courses.