Mediation analyses are crucial for understanding the mechanisms through which interventions or theoretical constructs influence outcomes within nested structures, commonly found in education, psychology, and sociology. Recognizing the importance of these effects, designing and analyzing robust studies to detect them is essential. This dissertation explored three topics regarding with the design and analysis of multilevel mediation effects under quasi-experimental designs (clustered regression discontinuity designs) and experimental designs (multisite designs), and empirical design parameters within the context of education. In the studies, we develop formulas to predict power and guide the design of clustered regression discontinuity and multisite studies probing mediation with ShinyApp software for easy implementations, and underscore the significance of drawing on mediator- and outcome-specific design parameter estimates when planning studies. [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.]