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Data Collectionmedium
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A researcher examines the relationship between college major and first-year salary using data from 8,000 recent graduates. She estimates two regression models:

Model 1 (Major only): Business majors earn 6,800morethanHistorymajors(956,800 more than History majors (95% CI: 6,800morethanHistorymajors(955,200–$8,400, p < 0.001).

Model 2 (Major + GPA): Business majors earn 2,400morethanHistorymajors(952,400 more than History majors (95% CI: 2,400morethanHistorymajors(95800–$4,000, p = 0.004).

Why does the estimated major effect decrease substantially when GPA is included?