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Mean, Median, and Mode Study Guide

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Updated on Oct 4, 2011

Introduction to Mean, Median, and Mode

Poor teaching leads to the inevitable idea that the subject (mathematics) is only adapted to peculiar minds, when it is the one universal science and the one whose… ground rules are taught us almost in infancy and reappear in the motions of the universe.

—HENRY J. S. SMITH, Irish mathematician (1826–1883)

This lesson focuses on three numbers that researchers often use to represent their data. These numbers are sometimes referred to as measures of central tendency. Translated to English, that simply means that these numbers are averages. This lesson defines mean, median, and mode; explains the differences among them; and shows you how to use them.

An average is a number that typifies or represents a group of numbers. You come into contact with averages on a regular basis—your batting average, the average number of pizza slices you can eat at one sitting, the average number of miles you drive each month, the average income for entry-level programmers, the average number of students in a classroom, and so forth.

There are actually three different numbers that typify a group of numbers:

  1. the mean
  2. the median
  3. the mode

Most of the time, however, when you hear people mention the average, they are probably referring to the mean. In fact, whenever this book uses the word average, it refers to the mean.

Let's look at a group of numbers, such as the number of students in a classroom at the Chancellor School, and find these three measures of central tendency.

Averages: Mean Median and Mode

Mean (Average)

The mean (average) of a group of numbers is the sum of the numbers divided by the number of numbers:

Average =

Example: Find the average number of students in a classroom at the Chancellor School.
Solution: Average =
  Average = 17

 

The average (mean) number of students in a classroom at the Chancellor School is 17. Do you find it curious that only two classrooms have more students than the average or that the average isn't right smack in the middle of things? Read on to find out about a measure that is right in the middle of things.

Median

The median of a group of numbers is the number in the middle when the numbers are arranged in order. When there is an even number of numbers, the median is the average of the two middle numbers.

Example: Find the median number of students in a classroom at the Chancellor School.
Solution: Simply list the numbers in order (from low to high or from high to low) and identify the number in the middle:

11 15 15 15 16 17 18 30

Tip

Did you know that median is the most common type of average used to measure the price of homes in the real estate market? The median is a helpful tool because it is protected against skewed data that is very far from the true center number. (Like a neighborhood where most of the houses cost around $200,000, but there's one house for $750,000 because it has a horse stable.)

Had there been an even number of classrooms, then there would have been two middle numbers:

9 11 15 15 16 17 18 30

With ten classrooms instead of nine, the median is the average of 15 and 16, or , which is also halfway between the two middle numbers.

If a number above the median is increased significantly or if a number below the median is decreased significantly, the median is not affected. On the other hand, such a change can have a dramatic impact on the mean—as did the one classroom with 30 students in the previous example. Because the median is less affected by quirks in the data than the mean, the median tends to be a better measure of central tendency for such data.

Consider the annual income of the residents of a major metropolitan area. A few multimillionaires could substantially raise the average annual income, but they would have no more impact on the median annual income than a few above-average wage earners. Thus, the median annual income is more representative of the residents than the mean annual income. In fact, you can conclude that the annual income for half the residents is greater than or equal to the median, while the annual income for the other half is less than or equal to the median. The same cannot be said for the average annual income.

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