# statsmodels.stats.power.normal_sample_size_one_tail¶

statsmodels.stats.power.normal_sample_size_one_tail(diff, power, alpha, std_null=1.0, std_alternative=None)[source]

explicit sample size computation if only one tail is relevant

The sample size is based on the power in one tail assuming that the alternative is in the tail where the test has power that increases with sample size. Use alpha/2 to compute the one tail approximation to the two-sided test, i.e. consider only one tail of two-sided test.

Parameters
difffloat

difference in the estimated means or statistics under the alternative.

powerfloat in interval (alpha, 1)

number of observations

alphafloat 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.

std_nullfloat

standard deviation under the Null hypothesis without division by sqrt(nobs)

std_nullfloat

standard deviation under the Alternative hypothesis without division by sqrt(nobs)

Returns
nobsfloat

sample size to achieve the desired power