# Sample Size Calculation and Sample Size Justification

Sample size calculation is concerned with how much data we require to make a correct decision on particular research.  If we have more data, then our decision will be more accurate and there will be less error of the parameter estimate.  This doesn't necessarily mean that more is always best in sample size calculation.  A statistician with expertise in sample size calculation will need to apply statistical techniques and formulas in order to find the correct sample size calculation accurately.

There are some basics formulas for sample size calculation, although sample size calculation differs from technique to technique.  For example, when we are comparing the means of two populations, if the sample size is less than 30, then we use the t-test.  If the sample size is greater than 30, then we use the z-test. If the population size is small, than we need a bigger sample size, and if the population is large, then we need a smaller sample size as compared to the smaller population.  Sample size calculation will also differ with different margins of error.

Statistical consulting provides a priori sample size calculation, to tell you how many participants you need, and sample size justification, to justify the sample you can obtain.  Knowing the appropriate number of participants for your particular study and being able to justify your sample size is important to meet your power and effect size requirements.  Using the appropriate power and establishing the effect size will tell you how many people you need to find statistically significant results.  Power and effect size measurements are also important to lending credibility to your study and are easily calculated by the experts at Statistics Solutions.

Sample size justification is as important as the sample size calculation.  If the sample size cannot be accurately justified, the researcher will not be able to make a valid inference.

Tools to Calculate Sample Size and Power Analysis

Statistics Solutions offers tools to calculate sample size for populations and power analysis for your dissertation or research study.  Our sample size for populations calculator is available for free with signup at our Basic Membership webpage.

The Sample Size/Power Analysis Calculator with Write-up is a tool for anyone struggling with power analysis.  Simply identify the test to be conducted and the degrees of freedom where applicable (explained in the document), and the sample size/power analysis calculator will calculate your sample size for a power of .80 of an alpha of .05 for small, medium and large effect sizes.  The sample size/power analysis calculator then presents the write-up with references which can easily be integrated in your dissertation document.  Click here for a sample.

For questions about these or any of our products and services, please email info@statisticssolutions.com or call 877-437-8622.

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