**Maximum likelihood estimation** (MLE) is a method that can be used to estimate the parameters of a given distribution.

This tutorial explains how to calculate the MLE for the parameter λ of a Poisson distribution.

**Step 1: Write the PDF.**

First, write the probability density function of the Poisson distribution:

**Step 2: Write the likelihood function.**

Next, write the likelihood function. This is simply the product of the PDF for the observed values x_{1}, …, x_{n}.

**Step 3: Write the natural log likelihood function.**

To simplify the calculations, we can write the natural log likelihood function:

**Step 4: Calculate the derivative of the natural log likelihood function with respect to λ.**

Next, we can calculate the derivative of the natural log likelihood function with respect to the parameter λ:

**Step 5: Set the derivative equal to zero and solve for λ.**

Lastly, we set the derivative in the previous step equal to zero and simply solve for λ:

Thus, the MLE turns out to be:

This is equivalent to the **sample mean** of the *n* observations in the sample.

**Additional Resources**

An Introduction to the Poisson Distribution

Poisson Distribution Calculator

How to Use the Poisson Distribution in Excel