The reliability of tests is an important characteristic as we use the test results for future decisions about the student’s educational advances for job selection and many more. The methods to assure the reliability of the tests have been discussed. Many examples have been provided in order to in-depth understanding of the concepts. Here we shall focus upon the different factors that may affect the reliability of the test. The degree of the effect of each factor varies from situation to situation. Controlling the factor may improve the reliability and otherwise, it may lower the consistency of production of scores. Some of the factors that directly or indirectly affect the test reliability are given as under.
Test Length
As a rule, adding more homogeneous questions to a test will increase the test’s reliability. The more observations there are of a specific trait, the more accurate the measure is likely to be. Adding more questions to a psychological test is similar to adding finer distinctions on a measuring tape.
Method Used to Estimate Reliability
The reliability coefficient is an estimate that can change depending on the method used to calculate it. The method chosen to estimate the reliability should fit the way in which the test will be used.
Heterogeneity of Scores
Heterogeneity is referred to as the differences among the scores obtained from class. You may say that there are some students who got high scores and some students who got low scores or intelligent students who got high scores and other ones who got low scores or the difference could be due to any reason such as income level, intelligence of the students, parents qualification etc. Whichever is the reason for the variability of the scores the greater the variability (range) of test scores, the higher the reliability. Increasing the heterogeneity of the examined sample increases variability (individual differences) thus reliability increases.
Difficulty
A test that is too difficult or too easy reduces the reliability (e.g., fewer test-takers get the answers correctly or vice-versa). A moderate level of difficulty increases test reliability.
Errors that Can Increase or Decrease Individual Scores
There might be some errors committed by the test developers that also affect the reliability of the tests developed by teachers. These errors initially affect the students’ scores, deviate the scores from the true ability of the students, and therefore affect the reliability. A careful consideration of these factors may help to measure the true ability of the students.
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The test itself: the overall look of the test may affect the student’s score. Normally a test is written in well readable font size and style, and the language of the test should be simple and understandable.
The test administration: After the development of the test, the test developer may have to prepare the manual of the test administration, the time, environment, invigilation, and anxiety also affect students’ performance while attempting the test. Therefore the uniform administration of the test leads to increased reliability.
The test scoring: Marking of the test is another factor in the variation in the scores of the students. Normally there are many raters to rate the students’ responses/answers on the test. Objective-type test items and the marking rubric for essay-type/supply-type test items help to get consistent scores.
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Ensuring the Reliability of Test
The most straightforward ways to improve a test’s reliability are
First, calculate the item-test correlations and rewrite or reject any that are too low. Any item that does not correlate with the total test at least (point-biserial) r = .25, should be reconsidered.
Second, look at the items that did correlate well and write more like them. The longer the test, the higher the reliability will be.
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