Confidence Interval

A confidence interval is a range that covers the true value at a given confidence level.

Formulas

x̄ ± t · s / √n

p̂ ± z · √(p̂(1 − p̂) / n)

The mean uses Student's t distribution and the proportion uses the normal approximation. A 95% interval does not mean 95% of the data lie inside it: the level describes the method.