A scientific study only counts as successful if it produces no false or misleading results. To keep errors to a minimum, researchers pay attention to measurement accuracy, known as reliability, from the moment they start designing an empirical study.
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General Definition of Reliability
Reliability (the dependability or consistency of a measurement) is a key quality criterion of any research project and describes how accurate the measurements in a study are. If the study is repeated at a different time under the same conditions, the researchers should arrive at the same or at least comparable results.
A study has high reliability when its findings are as free from random error as possible. If the measurements cannot be reproduced, the research is considered unreliable, which means its reliability is low.
Example of reliability:
A digital scale is a dependable measuring instrument with high reliability because it shows the same body weight every time, even when you step on it several times in a row. If researchers simply estimated their participants' weight by looking at them during an observation, the results would be far less consistent, so that method would not be considered very reliable.
How Reliability Is Tested in Research
In research practice, reliability can be estimated with several methods. The most common ones are:
Test-Retest Method
You repeat the measurement and compare the two rounds of data. If the same instrument gives the same results with the same participants both times, it is considered reliable. Because this method takes a lot of time and effort, it mainly pays off in larger studies.
Parallel Forms Method
Here you measure the same thing at the same time with two different but equivalent instruments, for example two versions of a questionnaire built around the same research question. If both versions produce comparable results, the instrument is considered reliable. The catch is that it is rarely easy to develop two truly comparable instruments for the same research question.
Split-Half Method
This is a variant of the parallel forms method. You split the items of a single instrument, such as a questionnaire, into two comparable halves and compare the scores the participants get on each half. If both halves lead to similar results, the instrument is considered reliable. The method is very popular because it only needs one round of data collection.
With all of these methods, the degree of reliability is calculated and expressed as a correlation coefficient: the closer it is to 1, the more reliable the instrument.
In smaller studies, especially in student papers, it is not always possible to test reliability with the methods above. As long as you choose a suitable research method (in other words, a suitable measuring instrument) and apply it carefully, that level of reliability is usually enough.