The "industrial" side of I–O psychology has its historical origins in research on individual differences, assessment, and the prediction of work performance. This branch of the field crystallized during World War I, in response to the need to rapidly assign new troops to duty stations. After the War, the growing industrial base in the US added impetus to I–O psychology. Walter Dill Scott, who was elected President of the American Psychological Association (APA) in 1919, was arguably the most prominent I–O psychologist of his time, although James McKeen Cattell (elected APA President in 1895) and Hugo Münsterberg (1898) were influential in the early development of the field. Organizational psychology gained prominence after World War II, influenced by the Hawthorne studies and the work of researchers such as Kurt Lewin and Muzafer Sherif.
See also: psychometrics and statistics
As described above, I–O psychologists are trained in the scientist–practitioner model. I–O psychologists rely on a variety of methods to conduct organizational research. Study designs employed by I–O psychologists include surveys, experiments, quasi-experiments, and observational studies. I–O psychologists rely on diverse data sources including human judgments, historical databases, objective measures of work performance (e.g., sales volume), and questionnaires and surveys. I–O researchers employ both quantitative and qualitative research methods. Quantitative methods used in I–O psychology include both descriptive statistics and inferential statistics (e.g., correlation, multiple regression, and analysis of variance). More advanced statistical methods employed by some I–O psychologists include logistic regression, multivariate analysis of variance, structural equation modeling, and hierarchical linear modeling (HLM; also known as multilevel modeling). HLM is particularly applicable to research on team- and organization-level effects on individuals. I–O psychologists also employ psychometric methods including methods associated with classical test theory (CTT), generalizability theory, and item response theory (IRT). In the 1990s, a growing body of empirical research in I–O psychology was influential in the application of meta-analysis, particularly in the area of the stability of research findings across contexts. The most well-known meta-analytic approaches are those associated with Hunter & Schmidt, Rosenthal, and Hedges & Olkin. With the help of meta-analysis, Hunter & Schmidt advanced the idea of validity generalization, which suggests that some performance predictors, specifically cognitive ability tests (see especially Hunter  and Hunter & Schmidt ) have a relatively stable and positive relation to job performance across all jobs. Although not unchallenged, validity generalization has broad acceptance with regard to many selection instruments (e.g. cognitive ability tests, job knowledge tests, work samples, and structured interviews) across a broad range of jobs. Qualitative methods employed in I–O psychology include content analysis, focus groups, interviews, case studies, and several other observational techniques. I–O research on organizational culture research has employed ethnographic techniques and participant observation to collect data. One well-known qualitative technique employed in I–O psychology is John Flanagan's Critical Incident Technique, which requires "qualified observers" (e.g., pilots in studies of aviation, construction workers in studies of construction projects) to describe a work situation that resulted in a good or bad outcome. Objectivity is ensured when multiple observers identify the same incidents. The observers are also asked to provide information about what the actor in the situation could have done differently to influence the outcome. This technique is then used to describe the critical elements of performance in...