StatCalc MD — Free Sample Size Calculator for Medical Research
StatCalc MD is a free online sample size and power analysis calculator designed for medical researchers, clinicians, and biostatistics students. All calculations run locally in your browser with no data collection.
26 Study Designs for Sample Size & Power Calculation
Means & t-tests
- Two independent means (parallel group RCT, continuous outcome)
- Paired / matched means (crossover, before-after)
- One-sample mean (vs. known reference value)
Proportions & Binary Outcomes
- Two independent proportions (RCT binary primary endpoint)
- One-sample proportion (vs. known p₀)
- Paired proportions — McNemar test
- Chi-square test (goodness-of-fit, independence)
- Non-inferiority trial — proportions
ANOVA Designs
- One-way ANOVA (k independent groups)
- Two-way ANOVA (factorial design with interaction)
- Repeated measures ANOVA (within-subjects)
- ANCOVA (covariate-adjusted group comparison)
Regression & Correlation
- Pearson correlation (H₀: ρ = 0, Fisher z-transform)
- Comparing two independent correlations
- Simple linear regression (slope test)
- Multiple regression R² (Cohen f²)
- Logistic regression (Hsieh, Bloch & Larsen method)
Survival Analysis
- Log-rank test (Schoenfeld 1981 formula)
- Cluster RCT — log-rank (survival with ICC adjustment)
- Cox proportional hazards regression
Clinical Trials & Equivalence
- Equivalence trial — TOST (two one-sided tests)
Diagnostic Studies
- ICC — intra-class correlation for reliability studies
Phase II Oncology
- Simon two-stage design (optimal and minimax)
Survey & Epidemiology
- Survey prevalence / proportion (with design effect DEFF)
- Survey mean estimation
SEM & Mediation
- Structural equation model — RMSEA-based power (MacCallum et al. 1996, semPower)
- Simple mediation — Sobel test
Key Features
- Manuscript paragraph generator — ready-to-paste sample size justification with embedded textbook citations and page numbers
- Verified replication code — R, Stata, and SAS commands to independently confirm every calculation
- 12 authoritative references — Cohen (1988), Aberson (2019), Machin et al. (2009), Chow et al. (2008), Zhang & Yuan (2018) WebPower, Jak et al. (2020) power4SEM, Moshagen & Bader (2024) semPower, Valliant et al. (2013), Aday & Cornelius (2006), Fleiss et al. (2003), Stata PSS Manual (2023), SAS PROC POWER (2015)
- WhatsApp share & PDF export of results
- Offline capable — works without internet after first load (Progressive Web App)
- iPhone optimised — mobile-first layout for use at the bedside
- No account required — completely free, no sign-in, no data collection
About
StatCalc MD was developed by Dr. Edimansyah Abdin, Principal Biostatistician at the Institute of Mental Health (IMH), Singapore. It is intended for use in clinical research protocol writing, ethics committee submissions, and biostatistics education.
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StatCalc MD — Free Sample Size Calculator