Validity and reliability are two key constructs in the analysis of the results produced by psychological studies. In fact, using more or less valid or reliable instruments can add or subtract value to a study itself. However, how do, validity and reliability differ?

In order to understand a little about the distinction between validity and reliability, you need to know what they are. Then, you can compare them.

Validity

There are different types of validity. However, when we refer to it in the field of psychometry, we refer to the degree to which the instrument measures the variable to be measured (Hernández-Sampieri, Fernández & Baptista, 2014).

For example, if a scale allows us to know the weight of an object, we can affirm that it’s valid to the extent that it fulfills the function for which it was designed. If we couldn’t make such a measurement, then the scale wouldn’t be valid for that specific task. Therefore, if an anxiety test that claims to measure you doesn’t perform, then it’s invalid.

On the other hand, in an investigation, the concept of validity “refers to what is true or what is close to the truth. In general, it is considered that the results of an investigation will be valid when the study is free of errors”(Villasís-Keever, 2018).

Determining validity is essential for all research. It means we’re able to trust the data obtained from that particular research design, the acquisition of data, the analysis of data processing, the interpretation, and its resulting judgment (Cohen, Manion, and Morrison, 2018).

Reliability

Reliability is understood as the consistency or stability of the measurements when a measurement process is repeated (Prieto and Delgado, 2010). It’s the degree to which an assessment tool produces consistent results by measuring the same thing under the same conditions.

A common procedure for obtaining the reliability of an instrument is to have it measure the same unchanging variable several times. For example, say we’re measuring the anxiety experienced by a subject. What’s usually done is to compare the measurement we obtain, with that given to us by an instrument that we know is reliable.

Let’s go back to the example of the scale. If we only weigh the object once, we can’t affirm that the measurement is reliable. That’s why we carry out several measurements. If we observe that we obtain very different values in each one, we can affirm that it’s not reliable.

The same happens in research and with all the instruments from which we obtain measurements, such as psychometric tests.

Differences between validity and reliability

Validity and reliability are closely related concepts. However, they have different purposes and focus on different aspects. Consequently, they provide different information on the quality of the research or measurement instrument. Let’s look at some of their differentiating characteristics:

1. Measurement

One of the main differences between validity and reliability is that the former measures the degree to which a research instrument measures what it’s intended to measure. However, reliability measures the degree to which the measuring or research instrument produces consistent results when repeated measurements are made.

2. Their associations

Validity relates to the correct or exact applicability of the instrument or investigation in a situation. Reliability is associated with the stability of test results.

3. The objective

Another difference between validity and reliability is that validity is focused on the result. On the other hand, reliability is concerned with the consistency of the result. This is regardless of whether the results are correct or not. In other words, there may be reliability without validity. Indeed, in an investigation or test, we may obtain consistent results, but this doesn’t imply that they’re correct. On the other hand, we can’t have correct results without them being reliable.

4. The research question

Validity asks the question “Does this instrument measure what should be measured?”. Reliability asks the question “How representative is this measurement? Can we trust it?”. Each question refers to a different object of study. Validity concerns precision and reliability relates to consistency or stability.

5. The evaluation

Validity and reliability differ in the way they’re evaluated. Reliability is assessed by checking that the results are consistent over time, between different observers, and between parts of the experiment itself. While validity is evaluated by checking how well the results correspond to established theories and other measures of the same concept.

The validity and reliability of an instrument are two different properties, but taken together they give us an idea of the quality of that instrument. The problem is that obtaining reliable and valid measuring instruments in psychology is especially expensive. This is due to the abstract nature of the medium. This cost makes it difficult to develop an instrument that doesn’t add a significant amount of error to the results of the research for which it’s used.

Nevertheless, the good news is that psychology has been fighting this battle for years, and has made remarkable progress to date.

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