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"The science and engineering workforce is composed of people who earned degrees over roughly three decades," the NSF explains on its website. "Because older cohorts of science and engineering workers are disproportionately white and male, women and minorities constitute a lower percentage of the overall science and engineering workforce than of science and engineering degree recipients who recently joined the workforce."
This finding is part of a larger study, "Women, Minorities, and Persons with Disabilities in Science and Engineering," which you can find here.