What Is Reliability in Psychology?

When we call someone or something reliable, we mean they are consistent and dependable. In psychology, reliability helps researchers ensure consistency in their work. When performing research, collecting data, and administering tests, researchers must ensure that the instruments they use are dependable. 

Reliability is also an important component of a good psychological test. After all, a test would not be very valuable if it was inconsistent and produced different results every time.

What Is Reliability in Psychology?

Reliability in psychology refers to the consistency of a measure. A test is considered reliable if we get the same result repeatedly.

It's a lot like expecting a bathroom scale to measure the same each time you use it. As long as your weight hasn't changed, you should get the same results each time you step on the scale. Because you keep getting the same results, the scale is reliable for measuring your weight. A reliable psychology test works in the same way.

For example, if a test is designed to measure a trait (such as introversion), then each time the test is administered to a subject, the results should be approximately the same. Unfortunately, it is impossible to calculate reliability exactly, but it can be estimated in a number of different ways.

Why Is Reliability So Important?

Reliability matters because we must trust the information that we get from psychological assessments and research.

These tools allow researchers to collect information about how people think, feel, and act. They also help doctors and mental health professionals evaluate and diagnose mental health conditions. We need these tools to be as consistent and accurate as possible.

Researchers are better able to trust the findings of their studies and experiments. Mental health professionals have more confidence in the accuracy of their diagnoses and treatments.

Types of Reliability

Psychologists can use several different methods in order to check for the reliability of a measure. Sometimes this involves administering a measure repeatedly with the same participants and checking for consistency. Sometimes it involves having different experts rate the results to gauge the consistency.

There are two main types of reliability: internal reliability and external reliability.

• Internal reliability means that the measure has consistency within itself. In other words, the same question posed differently would produce the same results. It is often measured using the split-half method.
• External reliability refers to how results compare to results between individuals and across time. It is often measured using test-retest, inter-rater, and parallel forms methods. 

Test-Retest Reliability

Test-retest reliability is a measure of the consistency of a psychological test or assessment. This kind of reliability is used to determine the consistency of a test across time. Test-retest reliability is best used for things that are stable over time, such as intelligence.

Test-retest reliability is measured by administering a test twice at two different points in time. This type of reliability assumes that there will be no change in the quality or construct being measured. In most cases, reliability will be higher when little time has passed between tests.

The test-retest method is just one of the ways that can be used to determine the reliability of a measurement. Other techniques that can be used include inter-rater reliability, internal consistency, and parallel-forms reliability.

Inter-Rater Reliability

This type of reliability is assessed by having two or more independent judges score the test. The scores are then compared to determine the consistency of the raters estimates.

One way to test inter-rater reliability is to have each rater assign each test item a score. For example, each rater might score items on a scale from 1 to 10. Next, you would calculate the correlation between the two ratings to determine the level of inter-rater reliability.

Another means of testing inter-rater reliability is to have raters determine which category each observation falls into and then calculate the percentage of agreement between the raters. So, if the raters agree 8 out of 10 times, the test has an 80% inter-rater reliability rate.

Parallel-Forms Reliability

Parallel-forms reliability is gauged by comparing two different tests that were created using the same content. This is accomplished by creating a large pool of test items that measure the same quality and then randomly dividing the items into two separate tests. The two tests should then be administered to the same subjects at the same time.

Internal Consistency Reliability

This form of reliability is used to judge the consistency of results across items on the same test. Essentially, you are comparing test items that measure the same construct to determine the tests internal consistency. It is often referred to as the split-half method of measuring reliability.

Because the two questions are similar and designed to measure the same thing, the test taker should answer both questions the same, which would indicate that the test has internal consistency.

Factors That Can Impact Reliability

There are a number of different factors that can have an influence on the reliability of a measure. First and perhaps most obviously, it is important that the thing that is being measured be fairly stable and consistent.

If the thing being measured is something that changes regularly, the results of the test will not be consistent.

Aspects of the testing situation can also affect reliability. For example, if the test is administered in a room that is extremely hot, respondents might be distracted and unable to complete the test to the best of their ability. This can have an influence on the reliability of the measure.

Reliability vs. Validity: What's the Difference?

It is important to note that just because a test has reliability it does not mean that it has validity. Validity refers to whether or not a test really measures what it claims to measure.

For example, imagine that job applicants are taking a test to determine if they possess a particular personality trait. ​While the test might produce consistent results, it might not actually be measuring the trait that it purports to measure.

So, what does it mean if a personality test is reliable? It means the test produces the same results each time a person takes it. While that might make that tool consistent, it doesn't necessarily mean the results are valid. Ideally, the measure would possess both reliability and validity, which means it consistently measures what it is supposed to measure.

How to Improve Reliability in Psychology Assessments

Improving the reliability of psychological assessment tools is important. If researchers find that a tool lacks sufficient reliability, there are a few ways they might go about improving the assessment's consistency.

• Develop standard procedures: Having clear test administration guidelines can often help improve reliability. This includes creating clear instructions, time limits, and other procedures that ensure the test is administered in the same way every time it is given.
• Train test administrators: The people who administer, rate, or score psychological tests should receive training in performing these tasks consistently.
• Consistent scoring criteria: How assessments are scored should be clear and consistent. Raters should have rubrics and guidelines to arrive at the same conclusions when assessing responses.

However, sometimes assessments remain unreliable for other reasons. In such cases, it is crucial for researchers to learn more about why the assessment is not producing reliable results.