# Statistical Power Analysis in Python¶

In this chapter we focus specifically on statistical power. We will use the NHANES dataset, so let’s first set that up.

```
import numpy as np
import pandas as pd
np.random.seed(12345)
from nhanes.load import load_NHANES_data
nhanes_data = load_NHANES_data()
adult_nhanes_data = nhanes_data.query('AgeInYearsAtScreening > 18')
adult_nhanes_data = adult_nhanes_data.dropna(subset=['WeightKg']).rename(columns={'WeightKg': 'Weight'})
```

## Power analysis¶

We can compute a power analysis using functions from the `statsmodels.stats.power`

package. Let’s focus on the power for an independent samples t-test in order to determine a difference in the mean between two groups. Let’s say that we think than an effect size of Cohen’s d=0.5 is realistic for the study in question (based on previous research) and would be of scientific interest. We wish to have 80% power to find the effect if it exists. We can compute the sample size needed for adequate power using the `TTestIndPower()`

function:

```
import scipy.stats
import statsmodels.stats.power as smp
import matplotlib.pyplot as plt
power_analysis = smp.TTestIndPower()
sample_size = power_analysis.solve_power(effect_size=0.5, power=0.8, alpha=0.05)
sample_size
```

```
63.765611775409525
```

Thus, about 64 participants would be needed in each group in order to test the hypothesis with adequate power.

## Power curves¶

We can also create plots that can show us how the power to find an effect varies as a function of effect size and sample size, at the alpha specified in the power analysis. We will use the `plot_power()`

function. The x-axis is defined by the `dep_var`

argument, while sample sizes (nobs) and effect sizes (effect_size) are provided as arrays.

```
#+
effect_sizes = np.array([0.2, 0.5, 0.8])
sample_sizes = np.array(range(10, 500, 10))
plt.style.use('seaborn')
fig = plt.figure()
ax = fig.add_subplot(1, 1, 1)
fig = power_analysis.plot_power(
dep_var='nobs', nobs=sample_sizes,
effect_size=effect_sizes, alpha=0.05, ax=ax,
title='Power of Independent Samples t-test\n$\\alpha = 0.05$')
#-
```

## Simulating statistical power¶

We can also simulate data to see whether the power analysis actually gives the right answer. We will sample data for two groups, with a difference of 0.5 standard deviations between their underlying distributions and a sample size based on power analysis, and we will then look at how often we reject the null hypothesis.

```
#+
num_runs = 5000
effectSize = 0.5
# perform power analysis to get sample size
power_analysis = smp.TTestIndPower()
sampleSize = power_analysis.solve_power(
effect_size=effectSize, power=0.8, alpha=0.05)
# round up from estimated sample size
sampleSize = np.int(np.ceil(sampleSize))
# create a function that will generate samples and test for
# a difference between groups using a two-sample t-test
def get_t_result(sampleSize, effectSize):
"""
perform a ttest on random data of n=sampSize
"""
group1 = np.random.normal(loc=0.0, scale=1.0, size=sampleSize)
group2 = np.random.normal(loc=effectSize, scale=1.0, size=sampleSize)
ttresult = scipy.stats.ttest_ind(group1, group2)
return(ttresult.pvalue)
# create input data frame for output
power_sim_results = pd.DataFrame({'p_value': np.zeros(num_runs)})
for run in range(num_runs):
power_sim_results.loc[run, 'p_value'] = get_t_result(sampleSize, effectSize)
p_reject = np.mean(power_sim_results['p_value'] < 0.05)
p_reject
#-
```

```
0.7994
```

This should return a number very close to 0.8.