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.  
 A: 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., $N$) required to achieve a given power.  You can search Cross Validated for more information about power analysis by clicking here: power-analysis, here: power, or possibly here sample-size.
Free software for power analyses:

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*G*Power

*PS: Power and sample size calculator

*Optimal Design software - statistical power analysis for multi-level and longitudinal research

Websites with free sample size calculators:

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*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:

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*R packages:

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*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.
