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The Reliability of Measurement: Definition, Importance & Types

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  • 0:05 Measurement
  • 1:28 Reliability
  • 3:01 Types
  • 4:41 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

Psychologists use tools like surveys and tests to measure psychological traits. But, what happens when a measurement tool is not consistent? In this lesson, we'll examine what reliability is, why it is important, and some major types.

Measurement

Jodie has noticed something about the people she encounters. It seems like the people she meets who have either a very large or small amount of education are not very racist. But, the people in the middle - those who have some advanced education but not a lot - seem to be racist. What's going on? Jodie wants to design a research study to see if this is normal. She wonders if people with racist attitudes have high, low, or medium educational levels on average.

In order to figure out the relationship between education and racism, Jodie has to measure her two variables. She can find out educational level pretty easily by asking people how much education they have or requesting that they provide transcripts from their high school or college.

But, how can Jodie measure people's attitudes about race? She can't look at someone and know what kind of attitude that person has, and she can't pull out a ruler, thermometer, or transcript and see how racist her subject is. Attitudes, like many psychological traits, are not visible on the surface.

Psychological measurement involves figuring out a way to assess a psychological trait. For example, Jodie might design a survey that asks people a bunch of questions about their thoughts on different things relating to racism. That is her measure.

Reliability

So, Jodie has her measure, or test, for racist attitudes. But, does her survey work?

Imagine for a moment that you have a bathroom scale, and every time you get on it, it gives you a different weight. One day it might say that you weigh 284 pounds, and the next day it might say that you weigh 115 pounds. In fact, you might get on the scale one minute and see that it says you weigh 136 pounds, and then the next minute you get on it, and it says that you weigh 207.

What's going on with your bathroom scale? It's not reliable at all. Reliability is the extent to which a measure gives consistent results. If your scale gives you a reasonably consistent reading every time you step on it, it is reliable.

Likewise, if Jodie gives her survey to a specific person two different times, and the results say that he is very racist on the first day, she wouldn't expect the results from the second test to say that he's not racist at all. All things being equal, a reliable measure gives the same answer.

Why is reliability important? Without it, we don't know what the truth is. If your bathroom scale gives a different weight every time you step on it, how will you know which is the correct weight? The answer is that you won't know.

If Jodie's survey is not reliable, how will she know if this person is racist and that one's not? Her test must be reliable or it doesn't give her the information she needs to draw conclusions about whether racism and educational level are related.

Types

There are many types of reliability:

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