Statistics & EBP

Reliability & Validity Explained | Statistics in Physiotherapy | Diagnosis

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Reliability & validity explained

Reliability & Validity Explained | Statistics in Physiotherapy | Diagnosis

While validity is the degree to which a test or tool measures what it claims to measure or in other words the accuracy of a test, reliability is the degree to which a test or tool produces similar results under consistent conditions or in other words the precision of a test. Good reliability of a test or tool is important because if the people who interpret the test cannot agree on the interpretation the test result will be of little use.

We can distinguish between intra-rater reliability and inter-rater reliability. Intra-rater reliability relates to the agreement between different measurements of one person and inter-rater reliability relates to the agreement between two or more raters. Now let’s look at both concepts of validity and reliability and how they relate to each other with the famous dartboard example: The bullseye represents the golden standard.

In scenario A, we hit the bull’s eye by chance so this dart was accurate, while all other darts are covered everywhere over the dartboard so we had bad precision or in other words poor reliability. An example could be a pivot-shift test for an ACL tear which is an accurate test that’s hard to perform. So if you are untrained it could be that your results are not consistent.

In scenario B we are precise or reliable because all of our shots land together but we are not accurate because we didn’t hit bull’s eye once. An example could be a scale that always measures 5 kilos too much. So our daily weighing would be pretty consistent, but not accurately display our actual weight.

In scenario C we are neither reliable nor accurate and in scenario D we are both reliable and accurate.

To sum it up, reliability is an important concept in statistics and without good reliability, the best clinical test in the world will be useless, because every test always relies to at least some degree on subjective interpretation by the observer. With training and with precise protocols, you will be able to improve the reliability with which you are performing clinical tests and measurements. Alright, so this was our video on reliability.

If you want to dig a little bit deeper into the topic and you want to know what the Kappa value is, click on the following link.



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Viera AJ, Garrett JM. Understanding interobserver agreement: the kappa statistic. Fam med. 2005 May 1;37(5):360-3.



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