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Power (sample size) calculators

Calculate how big your clinical trial needs to be with our easy to use online calculators.

There are four different sample size calculators - choose the correct one according to the type of clinical trial you are planning (superiority/equivalence) and the nature of the primary outcome variable (binary/continuous). A superiority trial is one where you want to demonstrate that one treatment or intervention is better than another (or better than no treatment/intervention). An equivalence trial is where you want to demonstrate that one treatment is no better or worse than an existing treatment.

Confused?

We can perform power calculations for you if you're not sure how to do it. Please contact us.

Continuous outcome equivalence trial

This calculator is designed for continuous outcomes (such as walking distance, blood pressure, white blood cell count) in parallel group equivalence trials. The mean outcome is compared between two randomised groups. You must define a difference between these means, d, within which you will accept that the two treatments being compared are equivalent.

Significance level (alpha)
Power (1-beta)
Standard deviation of outcome
Margin of equivalence, d

Technical note

Calculation based on the formula: n = f(α, β) × 2 × σ2 / d2
where σ is the standard deviation and f(α, β) = [Φ-1(α/2) + Φ-1(β)]2. Φ-1 is the cumulative distribution function of a standardised normal deviate

Adjustment for cross-overs based on formula: nadj = n × 10,000 / (100 - c1 - c2)2
where c1 and c2 are the percent cross-over in the control and experimental group respectively.

References

Pocock SJ. Clinical Trials: A Practical Approach. Wiley; 1983.
Julious SA. Sample sizes for clinical trials with Normal data. Statist. Med. 2004; 23:1921-1986