ABSTRACT
Clinical research validity depends critically on sound sampling methodology and adequate sample size determination, yet many published studies demonstrate deficiencies in these fundamental aspects. This educational review addresses sampling techniques and sample size calculations in clinical research. The review covers probability sampling approaches, including simple random, systematic random, stratified, and cluster sampling methods, contrasting these with non-probability techniques such as convenience, purposive, snowball, and quota sampling. For each method, we discuss implementation strategies, inherent biases, and appropriate clinical applications. Sample size determination principles are presented across multiple study designs, encompassing cross-sectional prevalence studies, case–control investigations, cohort studies, randomized controlled trials, and correlational analyses. Key statistical concepts, including Type I and Type II errors, statistical power, effect size estimation, and variance considerations, are also explained. Additionally, some available software tools for sample size calculation are outlined to facilitate implementation. This review ultimately provides clinical researchers with essential knowledge to make informed methodological decisions that enhance study quality and contribute to the evidence base for healthcare decision-making.