How to Calculate a Binomial Confidence Interval in Python

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The binomial confidence interval is used to derive the interval within which the probability of success on a binary outcome typically lies. The outcome could be anything from the chance of a user clicking on an ad to a patient responding to treatment, as long as the result is binary (e.g., success/failure, yes/no, true/false).

In Python, we have powerful libraries like SciPy, NumPy, and Statsmodels that we can use to calculate binomial confidence intervals with relative ease. This article will explain how you can calculate binomial confidence intervals using these libraries.

Libraries and Installation

To compute binomial confidence intervals in Python, we primarily need the following libraries:

  • NumPy: A fundamental package for numerical computation in Python.
  • SciPy: An open-source Python library used for scientific and technical computing.
  • Statsmodels: A Python library built specifically for statistics. It’s built on top of NumPy, SciPy, and Matplotlib.

You can install these libraries using pip:

pip install numpy scipy statsmodels

Understanding the Binomial Confidence Interval

The binomial confidence interval is based on the binomial distribution, which describes the number of successes in a fixed number of independent Bernoulli trials with the same probability of success.

The simplest method to calculate the binomial confidence interval is to use the normal approximation, which is applicable when the number of trials is large. The formula is:

CI = p̂ ± Z * sqrt((p̂*(1-p̂))/N)


  • CI is the confidence interval
  • p̂ is the sample proportion (successes / trials)
  • Z is the Z-score, which corresponds to the desired confidence level (e.g., 1.96 for a 95% confidence interval)
  • N is the number of trials

However, this method may not be accurate when the number of trials is small or the probability of success is close to 0 or 1. Other methods, such as the Wilson score interval or the Clopper-Pearson (exact) interval, may be more appropriate in these cases.

Calculating a Binomial Confidence Interval

Let’s see how we can calculate a binomial confidence interval in Python. We will first use the normal approximation, and then the exact method using the Statsmodels library.

First, let’s import the necessary libraries:

import numpy as np
import scipy.stats as stats
import statsmodels.api as sm

Let’s say we have the following data:

successes = 125
trials = 500

The sample proportion is:

p_hat = successes / trials

Normal Approximation

To calculate a 95% confidence interval using the normal approximation, we can use the following formula:

z = stats.norm.ppf(0.975)  # Z-score for a 95% confidence interval
margin_error = z * np.sqrt((p_hat*(1-p_hat))/trials)
confidence_interval = (p_hat - margin_error, p_hat + margin_error)

Here, we’re using the ppf() function from SciPy to get the Z-score that corresponds to a 95% confidence interval (the 0.975 quantile of the standard normal distribution).

Exact Method

To calculate a 95% confidence interval using the exact method (Clopper-Pearson), we can use the proportion_confint() function from the Statsmodels library:

confidence_interval = sm.stats.proportion_confint(successes, trials, alpha=0.05, method='binom_test')

Here, we’re specifying method='binom_test' to use the Clopper-Pearson method. Other available methods include ‘normal’ for the normal approximation, ‘wilson’ for the Wilson score interval, and ‘beta’ for the Bayesian confidence interval with a uniform prior.


In this article, we learned how to calculate a binomial confidence interval in Python. We started by discussing what a binomial confidence interval is and why it’s used, then we went over the necessary Python libraries and how to install them.

We learned how to calculate the binomial confidence interval using the normal approximation method, suitable for large samples and when the probability of success is not close to 0 or 1. We also learned how to calculate it using the exact method, which can be more accurate for small samples or probabilities of success near 0 or 1.

Remember, while the confidence interval provides a range of plausible values for the parameter of interest, it does not guarantee that the parameter lies within this range for every sample. Different samples can yield different confidence intervals, and it’s possible that some intervals will not contain the parameter. As with any statistical inference, conclusions drawn from confidence intervals should be made with a certain level of caution.

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