Before the late twentieth century, on the basis of studies showing widely varying validities for personnel selection techniques, the theory of "situational specificity" held sway. This principle holds that each organization, work setting and job is unique, requiring unique employee characteristics or skills, and that selection on any general ability yields little benefit. However, the development of statistical techniques such as meta-analysis allowed large samples to be assembled from smaller studies, revealing that apparent validity variations reflected statistical artifacts. Re-analysis, and subsequent studies, have established that general mental ability (IQ) predicts job performance across all jobs. The validity of IQ varies most notably with job complexity: IQ matters little for simple jobs such as packing, and a great deal in complex jobs such as medicine.