Concept of reliability in education

Reliability estimates evaluate the stability of measures, internal consistency of measurement instruments, and interrater reliability of instrument scores. 

Reliability can be operationally defined as the degree of correlation between alternative forms of a test, between halves, or between two performances. However, a more important definition takes into account the goals to be achieved, for example -the certainty that “true” results are not obscured by “random factors”.

It refers to the consistency of the tool of measurement used in the research.

Reliability tells you how consistently a method measures something. When we apply the same method to the same sample under the same conditions, we should get the same results. If not, the method of measurement may be unreliable.

The four commonly used methods of ensuring reliability include test-retest method, interrater method, internal consistency method, and parallel forms.

Types of reliability in education


The same test over time.


The same test conducted by different people.

Parallel forms

Different versions of a test which are designed to be equivalent.

Internal consistency

The individual items of a test.

Methods of estimate

Test-retest reliability

Test-retest reliability measures the consistency of results when you repeat the same test on the same sample at a different point in time. You use it when you are measuring something that you expect to stay constant in your sample.

For example -A test of colour blindness for trainee pilot applicants should have high test-retest reliability, because colour blindness is a trait that does not change over time.

Test-retest reliability can be used to assess how well a method resists these factors over time. The smaller the difference between the two sets of results, the higher the test-retest reliability. 

To measure test-retest reliability, we have to conduct the same test on the same group of people at two different points in time. Then we can calculate the correlation between the two sets of results.

Interrater reliability

Interrater reliability (also called interobserver reliability) measures the degree of agreement between different people observing or assessing the same thing. We use it when data is collected by researchers assigning ratings, scores or categories to one or more variables. To measure interrater reliability, different researchers conduct the same measurement or observation on the same sample.

Parallel forms reliability

Parallel forms reliability measures the correlation between two equivalent versions of a test. We generally use it when we have two different sets of questions designed to measure the same thing.

It is very important because if we want to use multiple different versions of a test (for example, to avoid respondents repeating the same answers from memory), we first need to make sure that all the sets of questions or measurements give reliable results.

Internal consistency

Internal consistency assesses the correlation between multiple items in a test that are intended to measure the same construct. We can easily calculate internal consistency without repeating the test or involving other researchers, so it’s a good way of assessing reliability when we only have one data set.

It is important because when we devise a set of questions or ratings that will be combined into an overall score, we have to make sure that all of the items really do reflect the same thing. If responses to different items contradict one another, the test might be unreliable.