# Calculating Sample Size for a Health Research Project: An Essential Step

Now we decided on the sampling technique, how do we calculate the sample size?

Sample size determination is a critical step in designing a health research project. It ensures that the study has sufficient statistical power to detect meaningful effects, increases the precision of the estimates, and enhances the generalizability of the findings. Calculating the appropriate sample size requires careful consideration of various factors, including the research objectives, study design, desired level of confidence, expected effect size, and variability in the outcome of interest. In this article, we will explore the key steps involved in calculating sample size for a health research project, along with an example to illustrate the process.

Step 1: Define Research Objectives and Study Design: First, clearly articulate the research objectives and identify the appropriate study design. Determine whether it is a cross-sectional study, case-control study, cohort study, or clinical trial, as the sample size calculation may differ for each design.

Step 2: Determine Desired Level of Confidence and Power: Choose the desired level of confidence (typically 95%) and power (typically 80%) for the study. The confidence level represents the probability of correctly detecting a true effect, while power indicates the probability of rejecting a null hypothesis when it is false.

Step 3: Estimate Effect Size and Variability: Estimate the effect size, which represents the magnitude of the difference or association you expect to observe. This can be based on previous studies, pilot data, or expert knowledge. Additionally, determine the variability of the outcome variable by considering the standard deviation or prevalence rate in the target population.

Step 4: Select Statistical Test: Identify the appropriate statistical test based on the research question and study design. Common tests include t-tests, chi-square tests, regression analysis, or survival analysis. The choice of test will influence the sample size calculation formula.

Step 5: Perform Sample Size Calculation: Use statistical software or sample size calculators to determine the required sample size based on the chosen test, desired level of confidence, power, effect size, and variability. The formula may vary depending on the statistical test selected.

Example: Let’s consider an example to illustrate the sample size calculation process. Suppose you are conducting a cross-sectional study to assess the prevalence of mental health disorders among conflict-affected populations. You want to estimate the prevalence with a margin of error no greater than 5%, a confidence level of 95%, and an expected prevalence of 30% based on previous studies. Using an online sample size calculator, you find that a sample size of 323 participants is needed.

Conclusion: Calculating the sample size for a health research project is crucial for ensuring the study’s validity and reliability. By following the steps outlined in this article and considering the specific research objectives, study design, effect size, variability, and statistical test, researchers can determine an appropriate sample size. Adequate sample size determination enhances the rigor and credibility of the study, enabling meaningful conclusions to be drawn and informing evidence-based healthcare practices.

References:

Bujang MA. A Step-by-Step Process on Sample Size Determination for Medical Research. Malays J Med Sci. 2021 Apr;28(2):15-27. doi: 10.21315/mjms2021.28.2.2. Epub 2021 Apr 21. PMID: 33958957; PMCID: PMC8075593.

Gupta KK, Attri JP, Singh A, Kaur H, Kaur G. Basic concepts for sample size calculation: Critical step for any clinical trials! Saudi J Anaesth. 2016 Jul-Sep;10(3):328-31. doi: 10.4103/1658-354X.174918. PMID: 27375390; PMCID: PMC4916819.