Relationship between Binomial and Poisson distributions

You just heard that the Poisson distribution is a limit of the Binomial distribution for rare events. This makes sense if you think about the stories. Say we do a Bernoulli trial every minute for an hour, each with a success probability of 0.1. We would do 60 trials, and the number of successes is Binomially distributed, and we would expect to get about 6 successes. This is just like the Poisson story we discussed in the video, where we get on average 6 hits on a website per hour. So, the Poisson distribution with arrival rate equal to \(np\) approximates a Binomial distribution for \(n\) Bernoulli trials with probability \(p\) of success (with \(n\) large and \(p\) small). Importantly, the Poisson distribution is often simpler to work with because it has only one parameter instead of two for the Binomial distribution.

Let's explore these two distributions computationally. You will compute the mean and standard deviation of samples from a Poisson distribution with an arrival rate of 10. Then, you will compute the mean and standard deviation of samples from a Binomial distribution with parameters \(n\) and \(p\) such that \(np = 10\).

This exercise is part of the course

Statistical Thinking in Python (Part 1)

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Exercise instructions

Using the rng.poisson() function, draw 10000 samples from a Poisson distribution with a mean of 10.
Make a list of the n and p values to consider for the Binomial distribution. Choose n = [20, 100, 1000] and p = [0.5, 0.1, 0.01] so that \(np\) is always 10.
Using rng.binomial() inside the provided for loop, draw 10000 samples from a Binomial distribution with each n, p pair and print the mean and standard deviation of the samples. There are 3 n, p pairs: 20, 0.5, 100, 0.1, and 1000, 0.01. These can be accessed inside the loop as n[i], p[i].

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

# Draw 10,000 samples out of Poisson distribution: samples_poisson


# Print the mean and standard deviation
print('Poisson:     ', np.mean(samples_poisson),
                       np.std(samples_poisson))

# Specify values of n and p to consider for Binomial: n, p



# Draw 10,000 samples for each n,p pair: samples_binomial
for i in range(3):
    samples_binomial = ____

    # Print results
    print('n =', n[i], 'Binom:', np.mean(samples_binomial),
                                 np.std(samples_binomial))

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