Reliability and Validity
These two terms are sometimes used interchangeably in research and evaluations. However, they mean different things. Reliability and validity are concepts used to evaluate the quality of research. They indicate how well a method, technique or test measures something. Reliability is about the consistency of a measure, and validity is about the accuracy of a measure.
It’s important to consider reliability and validity when you are creating your research design, planning your methods, and writing up your results, especially in quantitative research.
What is Reliability?
Reliability (or precision) refers to consistency. That is, if you use an instrument or test several times, you should get the same results. If the data (or the instrument) are unreliable, then the data are considered unrelated to the phenomenon or the concept being measured. This, therefore, means that the results cannot be repeated. For example, a broken thermometer that gives a different measurement every time it is placed in the same environment under the same conditions is not reliable.
What is the Validity?
Validity simply means that a test or instrument is accurately measuring what it’s supposed to. In evaluations, we usually refer to two types of validity; internal and external.
Internal validity refers to the extent to which an instrument (or an evaluation) correctly answers the questions it claims to answer about what is being tested (or evaluated). For example, a questionnaire (instrument) that asks persons to state the amount of their donations. Is the answer an indication of how charitable people are? Or is it their disposable income that is actually being measured by this instrument?
External validity refers to the extent to which the results of an evaluation can be generalised to other situations. That is, the extent to which the sample selection reflects the population. The value of external validity is the ability to generalise the results to a larger population.
A Final Word...
Tests or instruments that are valid are also reliable. E.g. a properly functioning thermometer is valid (and reliable) because it measures the correct temperature in a consistent manner every time. However, tests or instruments can be reliable but not always valid. E.g. , the broken thermometer that is a degree off would be reliable (giving you the same results each time) but not valid (because it was not recording the correct temperature).
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