Reliability is the degree of consistency between two measures of the same thing. It checks if the same score is obtained or not when somebody measure same thing more than once. A test can be considered reliable when it produces the same repeated result under the same conditions.

Thus, reliability is related to repeatability or stability of findings or results of a particular test no matter how many times it has been measured. Hence, if a test is conducted for the second time, will it provide the same output and if the answer is yes, the data is reliable.

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For example – a person weighs his weight on a weighing machine. The reliable machine is supposed to show same (with slight up and down) reading (within a certain period) again and again regardless of number of times the readings are taken. The slight change in reading can be considered for a reliable machine but what if once it shows 60kgs and then in second and third reading, it shows 65 and 63 kgs respectively. It cannot be a reliable data and hence machine is not reliable.

Reliability Coefficient

Reliability can be estimated statistically by calculating the correlation coefficient. Reliability coefficient quantifies the degree of consistency. There are chances of presence of errors (e.g., errors in assessment) which may produce inconsistent test results. Reliability coefficient can be helpful in understanding such errors in testing.

In the absence of errors in the assessment process, same test score can be achieved repeatedly and this will be the "true" score. Thus, in case of reliable test/experiment, high positive correlation between repeat scores should exist.

Types of reliability

Split-half method

To check if all the questions/items of a test are contributing equally to the final score, Split-half method can be used. Split-half method is one of the approach to measures the internal reliability of an experiment by estimating the correlation between the two set of scores. In this method, all the questions of an experiment are splitted in two halves. Questions are divided in such a way that any two questions which are intended to examine the same aspect, fall into different halves. These two sets of questions will be introduced to a group of individuals at the same time. Now, the total score for each set will be calculated and the results of one half of the test (e.g., even numbered questions) will be compared with the results of the other half (e.g., odd numbered questions). If the two halves of the test provide similar results this would suggest that the test has internal reliability.

Test-retest method

The test-retest method assesses the external consistency of a test. Basically, it checks the stability of the test by conducting same test for the participant on two different points of time. If the same/similar results are obtained then external reliability is established.

Example – An instrument/individual takes a test and records the first reading. In a different occasion same process is repeated under same/similar conditions and readings are taken. The obtained correlation coefficient of the readings will determine the consistency of the measures.

Time is an important factor in this test. If the time gap is too short then participants/instruments may recall the answers from first test which could bias the results. On the other hand, if the time gap is too long there are possibilities that the participants/instruments could have changed in some way which could also bias the results.

Inter-rater method

This method is used to assess the degree to which different raters/observers give consistent estimates of the same phenomenon. For example – a group of researchers study the same behaviour (e.g., social skills of college students) independently in order to avoid bias factor. At the end of study, all researchers present their observations. These observations are compared and if the data is similar then it is reliable.