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It has proven difficult to demonstrate the relationship between wage differentials and differences in worker efficiency. This study examines relationships between such differentials and worker proficiency in a clerical labor market. The author concludes that a statistically significant proportion of starting salary variation is attributable to proficiency differentials, although numerous other factors undoubtedly also play significant roles. (Author's abstract courtesy EBSCO.)
Eaton H. Conant (1963) studied this question.