School systems across the United States increasingly use performance-based assessments (PBAs) as alternatives to traditional standardized tests, like the SAT, to make post-secondary and workforce readiness (PWR) determinations. However, very little research has been conducted to validate such alternative assessments as valid indicators of a student's readiness for life after high school. This study attempts to fill this gap in the literature by investigating the validity of one rural Colorado high school's use of a PBA (in the form of a capstone project) to make PWR determinations (between the school years of 2020-21 and 2022-23). I investigate the validity of this assessment for determining PWR skill mastery by examining three assumptions. These assumptions target fairness, scoring, and how the scores from the summative capstone event relate to other PWR indicator variables and post-secondary outcomes. I find that, while the school has made meaningful shifts towards establishing a curriculum that is coherent with the capstone project graduation requirement, opportunities to be adequately supported to complete the capstone project do not appear to be evenly distributed across students. The scoring procedures also do not appear appropriate, applied as intended, or well-designed to limit rater biases from impacting the scores students receive. While scores on the summative capstone event are associated with other variables indicative of PWR and college enrollment and persistence, it does not predict these outcomes over and above the SAT. Furthermore, high school GPA stands out as the strongest predictor of college enrollment and persistence. These findings underscore the importance of curricular coherence and clearly defined scoring procedures when using a PBA for high-stakes purposes. They also point to the need for more research on how official high school GPA associates with college outcomes. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]