Estimate, Plan a Study and Find Probabilities

Use these calculators to report how precise an estimate is and to plan a study before collecting data. The confidence interval, effect size and probability calculators work on numbers you already have; the sample size and power calculators work out how many observations a study needs. The binomial and Poisson calculators give exact probabilities for counts of events.

Tools in this group

• Binomial Distribution Calculator — Enter n, p and x to get any binomial probability — exactly, at most, at least or between — with the table and chart.
• Confidence Interval Calculator — Enter your sample numbers and a confidence level to get the confidence interval, margin of error and critical value for a mean or a proportion.
• Effect Size Calculator (Cohen's d) — Enter two groups' means, SDs and sizes, or a t statistic, to get Cohen's d, Hedges' g and Glass's Δ with a confidence interval.
• Poisson Distribution Calculator — Enter the average rate λ and a count x to get the exact or cumulative Poisson probability, with the table and chart.
• Power Analysis Calculator — Pick a test and solve for power, sample size or the smallest detectable effect; the power curve shows how power grows with n.
• Sample Size Calculator — Set the confidence level and margin of error (or the difference and power) to get the sample size you need, rounded up, with the formula.