Statistics · Quick Reference

Mean Cheat Sheet

Statistics · Lesson 2/11

In one line: the mean, or average, is the most common measure of central tendency — a single number that summarizes an entire dataset's typical value.

Key Ideas

1Mean Formula. Add up every value in the dataset, then divide by how many values there are.

2Population Mean vs. Sample Mean. The population mean is written mu (calculated from the entire population); the sample mean is written x-bar (calculated from a sample).

3Sensitivity to Outliers. The mean is heavily influenced by extreme values — a single very large or very small number can pull the mean away from what feels 'typical'.

4Weighted Mean. Used when values contribute unequally; multiply each value by its weight, sum the results, then divide by the total weight.

5When to Use the Mean. Best for symmetric datasets without extreme outliers; for skewed data, the median is often more representative.

Worked Examples

Find the mean of 4, 8, 6, 10, 12.

A class of 5 students scores 60, 70, 80, 90, and 100 on a quiz. Find the mean score.

Add an outlier of 500 to the dataset 10, 12, 14, 16 and see how the mean changes.

New mean = 110.4, dramatically pulled up by the outlier

Formulas

Mean = (sum of all values) / (number of values)

Practice Yourself

Find the mean of 2, 4, 6, 8.

Find the mean of 15, 20, 25.

Find the mean of 100, 200, 300, 400, 500.

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