Psychological researchers use statistical methods to determine if certain treatments are effective, and clinical psychologists must be able to interpret statistical data to interpret diagnostic material and studies. Psychologists working with groups also rely on statistics for measurements.
As with all sciences, psychology is partially based on a mathematical foundation. Hypotheses need to be tested, and statistical analysis provides a means of determining whether treatments appear to be effective or not. Psychologists working with patients must read studies to determine what scientific literature shows to be most effective, and they must have a strong understanding of statistics to do so.
Diagnosing problems is an essential part of clinical psychology, but the results rarely show up on brain scans or other tools. As a result, psychologists rely on surveys and information provided by patients. Psychologists must be able to interpret these results to make the best diagnosis possible. Understanding the prevalence of various problems also helps.
Psychologists often work with groups of people in schools, offices and other organizations, and much of their work requires them to locate and measure various trends. However, they must be able to determine whether trends are real effects or just statistical noise. Again, statistics play a key role, and using p-values and Bayesian statistical methods allows psychologists to make important observations.