# statsmodels.stats.power.TTestIndPower.solve_power¶

TTestIndPower.solve_power(effect_size=`None`, nobs1=`None`, alpha=`None`, power=`None`, ratio=`1.0`, alternative=`'two-sided'`)[source]

solve for any one parameter of the power of a two sample t-test

for t-test the keywords are:

effect_size, nobs1, alpha, power, ratio

exactly one needs to be `None`, all others need numeric values

Parameters:
effect_size`float`

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

nobs1

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`

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.

power`float` `in` `interval` (0,1)

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.

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`str`, ‘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’.

Returns:
value`float`

The value of the parameter that was set to None in the call. The value solves the power equation given the remaining parameters.

Notes

The function uses scipy.optimize for finding the value that satisfies the power equation. It first uses `brentq` with a prior search for bounds. If this fails to find a root, `fsolve` is used. If `fsolve` also fails, then, for `alpha`, `power` and `effect_size`, `brentq` with fixed bounds is used. However, there can still be cases where this fails.

Last update: May 17, 2023