NormalIndPower.power(effect_size, nobs1, alpha, ratio=1, alternative='two-sided')[source]

Calculate the power of a t-test for two independent sample


effect_size : float

standardized effect size, difference between the two means divided by the standard deviation. effect size has to be positive.

nobs1 : int or float

number of observations of sample 1. The number of observations of sample two is ratio times the size of sample 1, i.e. nobs2 = nobs1 * ratio ratio can be set to zero in order to get the power for a one sample test.

alpha : float in interval (0,1)

significance level, e.g. 0.05, is the probability of a type I error, that is wrong rejections if the Null Hypothesis is true.

ratio : float

ratio of the number of observations in sample 2 relative to sample 1. see description of nobs1 The default for ratio is 1; to solve for ratio given the other arguments it has to be explicitly set to None.

alternative : string, ‘two-sided’ (default), ‘larger’, ‘smaller’

extra argument to choose whether the power is calculated for a two-sided (default) or one sided test. The one-sided test can be either ‘larger’, ‘smaller’.


power : float

Power of the test, e.g. 0.8, is one minus the probability of a type II error. Power is the probability that the test correctly rejects the Null Hypothesis if the Alternative Hypothesis is true.