Login

Finding Z-Scores: Definition & Examples

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Estimating Areas Under the Normal Curve Using Z-Scores

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
 Replay
Your next lesson will play in 10 seconds
  • 0:05 What is a Z-Score?
  • 1:03 Review of Standard Deviation
  • 2:18 Why Use Z-Scores?
  • 3:31 How to Calculate a Z-Score
  • 4:27 Examples
  • 5:55 Lesson Summary
Add to Add to Add to

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Login or Sign up

Timeline
Autoplay
Autoplay
Create an account to start this course today
Try it free for 5 days!
Create An Account

Recommended Lessons and Courses for You

Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

Talking about multiples of standard deviations can get exhausting and confusing. Luckily, z-scores allow us to talk about how far a point is removed from a mean in terms of how many standard deviations away it is.

What Is a Z-Score?

Imagine that you were dealing a data set that had a standard deviation of 2. Remember that standard deviation is simply a measure of how scattered a collection of data is from the mean. Obviously, much of your data will be the mean plus or minus 2. Even more will be two standard deviations away from the mean, so the mean plus or minus 4. That notation works pretty well when the standard deviation is succinct. After all, 2 is a pretty easy concept to understand.

But what if your standard deviation was a very small decimal, as it often is in engineering? Or a less-simple number, like 593, as it could be in polling. Wouldn't it be nice to have a unit that measures the value of something in terms of standard deviations away from the mean? Luckily, we have exactly such a tool: the z-score. The z-score is a measure of distance from the mean in terms of how many standard deviations it is removed from the mean.

Review of Standard Deviation

But wait, how do you find the standard deviation? Chances are you may already remember, but if you don't, we'll review that quickly. First, take the mean of all the values in the set. Then, subtract the mean from each value, then square each difference. Find the mean of the sum of those differences, then take the square root of that number.

For example, let's say you wanted to know the population standard deviation of the following test scores: 98, 90, 86, 83, and 70. First, take the mean, which comes out to 85.4. Then subtract that from each score. You get the following values: 12.6, 4.6, 0.6, -2.4, and -15.4. Square each of those, giving you 158.76, 21.16, 0.36, 5.76, and 237.16. Add all those up and find the average of them, which turns out to be 84.64. Then take the square root, which is 9.2.

Why Use Z-Scores?

By far, the biggest advantage of a z-score is its brevity. When I say to include all values between z-scores of -2 and 2, you know that I want to include 95% of values. That's because within two standard deviations on either side of the mean are 95% of the data in a set.

It would be pretty easy to do that with values when the standard deviation is a highly regular number, like 2, but could you imagine doing that when the standard deviation is 0.00487? Or when it is 1,381? You could bring out a calculator and find that two standard deviations of 0.00487 is 0.00974, and the two of 1,381 is 2,762. However, that introduces a great deal of potential errors. Z-scores limit those errors by giving a concise answer. With z-scores, you can worry about other things, not making sure that 0.00974 is exactly double of 0.00487.

How to Calculate a Z-Score

As calculating a z-score is simply finding something in terms of the standard deviation, it is a relatively simple formula. First, subtract the mean from the value in question. Then, divide the answer by the standard deviation. If you have a value in question of 55, a mean of 30, and a standard deviation of 20, you would take 55 and subtract 30. That leaves 25. Divide 25 by the standard deviation of 20 and you end up with 1.25. 1.25 is the z-score of that data point.

Sometimes, that number may come out negative. That's fine; negative numbers just mean that the z-score in question is less than the mean. On a normal distribution curve, that means that it is to the left of the high point of the curve. Likewise, positive numbers are always on the right of the curve.

To unlock this lesson you must be a Study.com Member.
Create your account

Register for a free trial

Are you a student or a teacher?
I am a teacher
What is your educational goal?
 Back

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back

Earning College Credit

Did you know… We have over 95 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it free for 5 days!
Create An Account
Support