How to Calculate a Trimmed Mean (Step-by-Step)


A trimmed mean is the mean of a dataset that has been calculated after removing a specific percentage of the smallest and largest values from the dataset.

To calculate a X% trimmed mean, you can use the following steps:

Step 1: Order each value in a dataset from smallest to largest.

Step 2: Remove the values in the bottom X% and top X% of the dataset.

Step 3: Calculate the mean of the remaining values.

The following examples show how to calculate a trimmed mean in practice.

Example 1: Calculate a 10% Trimmed Mean

Suppose we have the following dataset:

Dataset: 4, 8, 12, 15, 9, 6, 14, 18, 12, 9

Here is how to calculate the 10% trimmed mean for the dataset:

Step 1: Order each value in a dataset from smallest to largest.

Ordered Dataset: 4, 6, 8, 9, 9, 12, 12, 14, 15, 18

Step 2: Remove the values in the bottom 10% and top 10% of the dataset.

There are 10 total values in the dataset. Thus, 10% * 10 = 1. This means we need to remove the one smallest value and one largest value from the dataset:

Trimmed Dataset: 6, 8, 9, 9, 12, 12, 14, 15

Step 3: Calculate the mean of the remaining values.

10% trimmed mean = (6+8+9+9+12+12+14+15) / 8 = 10.625

The 10% trimmed mean is 10.625.

Example 2: Calculate a 20% Trimmed Mean

Suppose we have the following dataset:

Dataset: 22, 25, 29, 11, 14, 18, 13, 13, 17, 11, 8, 8, 7, 12, 15, 6, 8, 7, 9, 12

Here is how to calculate the 20% trimmed mean for the dataset:

Step 1: Order each value in a dataset from smallest to largest.

Ordered Dataset: 6, 7, 7, 8, 8, 8, 9, 11, 11, 12, 12, 13, 13, 14, 15, 17, 18, 22, 25, 29

Step 2: Remove the values in the bottom 20% and top 20% of the dataset.

There are 20 total values in the dataset. Thus, 20% * 20 = 4. This means we need to remove the four smallest values and four largest values from the dataset:

Trimmed Dataset: 8, 8, 9, 11, 11, 12, 12, 13, 13, 14, 15, 17

Step 3: Calculate the mean of the remaining values.

20% trimmed mean = (8+8+9+11+11+12+12+13+13+14+15+17) / 12 = 11.9167

The 20% trimmed mean is 11.9167.

Bonus Resource: Trimmed Mean Calculator

If you have an extremely large dataset and do not want to calculate a trimmed mean by hand, feel free to use this Trimmed Mean Calculator.

For example, here’s how to use this calculator to find the 20% trimmed mean from the previous dataset:

Notice that the 20% trimmed mean matches the value that we calculated by hand.

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