Internal consistency reliability is a measure of how well the items on a test measure the same construct or idea. Learn more about internal consistency reliability from examples and test your knowledge with a quiz.

What Is Reliability?

Suppose that you are a psychologist. A client walks into your office and tells you that she has been having nightmares and scary thoughts. She explains that her symptoms started a few days after she survived a car accident. She also reports having flashbacks of her car accident every time she tries to drive a car. She has been taking the bus and relying on friends to take her everywhere she needs to go.

After talking with her for a bit longer, you notice that the symptoms that she is describing sound a lot like post-traumatic stress disorder (PTSD). You decide to search for a psychological test that is designed help you to detect and diagnose PTSD.

You come across the Posttraumatic Stress Diagnostic Scale (PDS). The test manual indicates that the PDS is a valid measure of PTSD. This means that the PDS is accurately able to measure PTSD. You begin to wonder about the consistency, or reliability, of the PDS results. That is, will the PDS produce the same results over and over again?

There are four types of reliability that you can explore:

  • Inter-rater reliability: In instances where there are multiple scorers or ‘raters’ of a test, the degree to which the raters’ observations and scores are consistent with each other
  • Parallel forms reliability: In instances where two different types of a measurement exist, the degree to which the test results on the two measures is consistent
  • Test-retest reliability: The degree to which the measurement’s results are consistent over time
  • Internal consistency: The degree to which every test item measures the same construct

Internal Consistency Reliability Defined

Internal consistency is a method of reliability in which we judge how well the items on a test that are proposed to measure the same construct produce similar results. For example, a question about the internal consistency of the PDS might read, ‘How well do all of the items on the PDS, which are proposed to measure PTSD, produce consistent results?’ If all items on a test measure the same construct or idea, then the test has internal consistency reliability.

For example, suppose you wanted to give your clients a 3-item test that is meant to measure their level of satisfaction in therapy sessions. The first item is ‘You almost always feel satisfied after your therapy sessions’. The second item is ‘You almost always enjoy therapy’. The third item is ‘You almost never feel satisfied with therapy’. If a client agrees with the first two items and disagrees with the third, the test has good internal consistency.

Methods of Measurement

There are three methods used to measure internal consistency reliability:

  • Cronbach’s alpha: The most commonly used measurement of internal consistency.
  • Split-halves test: Involves splitting the test items in half (i.e., forming a group of all even items and another group with all of the odd items) and correlating the two halves.
  • Kuder-Richardson test: Similar to the split-halves test. You find the average correlation for all of the possible split-half combinations.

No matter which method you use, the closer your measurement is to 1, the higher your internal consistency is.


You look in the test manual of the PDS and find that Cronbach’s alpha is 0.91. This indicates that the PDS has strong internal consistency.

Suppose that you have a 10-item test. You divide the test into the following two groups:

  • Group A: Items 2, 4, 6, 8 and 10
  • Group B: Items 1, 3, 5, 7 and 9

You calculate the correlation between the two halves and find that it is 0.80. Your test has strong internal consistency.

Lesson Summary

Internal consistency reliability is one of four types of reliability. Internal consistency reliability refers to the degree to which test items measure the same construct. There are three measures of internal consistency reliability:

  1. Cronbach’s alpha
  2. Split-half test
  3. Kuder-Richardson test

Learning Outcomes

After you have finished, you should be able to:

  • Recall what reliability is
  • Name the four types of reliability
  • Explain what internal consistency reliability is
  • State the three methods for measuring internal consistency reliability
  • Understand how to determine how strong a test’s internal consistency is