The aim of this dissertation was to develop and validate a novel Epistemic Competence Beliefs Measure (ECBM) in order to explore students' ability to evaluate the characteristics of scenarios about complex and controversial social issues. Epistemic competence was hypothesized to be critical in performing multiple source use (MSU) tasks and in the writing of argumentative essays. The study was conducted in two phases, focusing on the ECBM's development, its content validity, and predictive validity in an ecological-valid MSU task that was embedded in an undergraduate course on learning. Phase 1 of the study involved creating the ECBM based on the literature about epistemic beliefs and epistemic cognition. The ECBM consisted of eight scenarios addressing socially relevant controversial issues presumed to vary in terms of complexity, controversiality, relevance to the self and others, and the domains relevant to their resolution. Each scenario was followed by questions that asked about those dimensions. To establish content validity, the scenarios and questions were evaluated by a panel of international experts. In Phase 2, the ECBM was administered to college students prior to their completion of a MSU task. Data sources included argumentative claim selection, search logs, notes, essays, and a retroactive behavior questionnaire. Data were analyzed using content analysis, cluster analysis, ANOVA, multiple linear regression, and regression trees to determine predictive validity. The results revealed significant variability in students' espoused epistemic competence beliefs, although no direct predictive relation to enacted epistemic competence was established. Students' critical-analytic thinking on the argumentative essay varied significantly by their GPA and relational reasoning ability. Notably, students with higher relational reasoning scores exhibited superior critical-analytic thinking in their essays. For future research, the ECBM can be refined and more closely integrated into the MSU project. Further, diversifying study populations across different sociocultural contexts and employing Bayesian and mixed-methods analyses can provide deeper insights into epistemic competence and critical-analytic thinking. The results also suggest that students should be introduced to the construct of epistemic competence and its importance, and reminded of the value of MSU tasks and the characteristics of quality argumentative essays. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]