Fourteen calculators for sample size, standard deviation, p-values, confidence intervals and probability. Each result page names the exact formula behind it, sample SD or population SD, a z, t or chi-square p-value, so you know which method produced your number before you cite it.
Everything from study design to hypothesis testing.
Required sample size for surveys and A/B tests.
Open calculator →Mean, variance and SD for a data set.
Open calculator →P-values from z, t or chi-square statistics.
Open calculator →CIs for means and proportions.
Open calculator →Normal, binomial and Poisson probabilities.
Open calculator →The standard deviation calculator returns both the sample figure (divided by n minus 1) and the population figure (divided by n) from the same data set, so you are not left guessing which one your assignment or your dataset actually calls for.
The probability calculator sums the real factorial-based probability mass function for binomial and Poisson cases instead of leaning on a normal approximation, which matters most at the small sample counts where a bell curve stops being a fair stand-in.
Feed the p-value calculator a z, t or chi-square statistic and it switches methods: the normal CDF for z, the incomplete beta function for t, the incomplete gamma function for chi-square. The page tells you which one ran.
Paste 4, 8, 6, 5, 3, 7 into the standard deviation calculator. Population SD comes back 1.71; sample SD comes back 1.87. Nothing rounds those two apart, the only thing that changed is whether you are treating those six values as the whole population or as a sample pulled from something bigger, and that single choice moves the divisor from 6 to 5.