This module demystifies sample size calculation. You will learn about the three key statistical inputs—confidence level, margin of error, and population variance—that determine a valid sample size. Through hands-on practice with an in-browser calculator, you will master the skill of determining the minimum number of participants needed for a statistically reliable study. Sample size decisions are not purely mathematical. In professional research settings, they balance statistical rigor with cost, timeline, and access to respondents. In academic research, probability sampling is often required to support generalizable claims. In business and market research, however, method choice is guided by decision context, constraints, and intended use of the findings. This course focuses on professional research design, where the goal is actionable insight—not publication.