# Free internet or downloadable resources for sample size calculations

Today I noticed this question, and I thought it would be helpful if we had a thread that listed resources that people could conveniently access for power analysis / sample size calculations, perhaps analogous to this thread: Resources for learning R.

## Answer

Power analysis refers to analytical procedures that attempt to determine the power of a statistical test (i.e., the probability of rejecting a false null hypothesis) or the sample size (i.e., $$NN$$) required to achieve a given power. You can search Cross Validated for more information about power analysis by clicking here: , here: , or possibly here .

Free software for power analyses:

Websites with free sample size calculators:

• Russ Lenth has a java-based web app
• Michael Friendly has a power analysis / sample size calculator for ANOVA designs
• Rollin Brant’s webpage for comparing two proportions
• OpenEpi has sample size/power calculators for basic epidemiological study designs.
• webpower a web based interface for power analyses (based on the package listed below) for people who don’t use R

Open source programming languages with power analysis libraries / modules:

• R packages:

• pwr is the oldest power-analysis library; some introductory info can be found on Quick-R
• PoweR: Computation of power and level tables for hypothesis tests
• Power2Stage: Power and Sample size distribution of 2-stage BE studies via simulations
• powerAnalysis: Power analysis in experimental design
• powerGWASinteraction: Power Calculations for Interactions for GWAS
• powerMediation: Power/Sample size calculation for mediation analysis, simple linear regression, logistic regression, or longitudinal study
• powerpkg: Power analyses for the affected sib pair and the TDT design
• powerSurvEpi: Power and sample size calculation for survival analysis of epidemiological studies
• PowerTOST: Power and Sample size based on two one-sided t-tests (TOST) for (bio)equivalence studies
• longpower: Power and sample size for linear model of longitudinal data
• webpower: “a collection of tools for conducting both basic and advanced statistical power analysis” (also a website)
• There is a Python module, statsmodels, that can run some power analyses. There is some information here.

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