Validity and reliability--what's it all about? Part 2 reliability in quantitative studies.

TA X I In the last update (November 2004) we considered validity in quantitative studies: whether a tool measures what it sets out to measure. Reliability refers to the consistency, stability and repeatability of results. Testing for reliability ensures that consistent results would be obtained in identical situations on different occasions. It is possible for a measure to be reliable but not valid. The example often cited to explain the difference between validity and reliability is that of a clock that is five minutes fast. The clock is reliable because it is always five minutes fast. However, it is not valid because it does not conform to a recognised standard such as Greenwich Mean Time. To give a clinical example, the weight of an infant on one set of scales may not be the same on a different set of scales – the scales themselves are reliable, but set to a different standard. Or, if one nurse weighs the baby on the same set of scales as another nurse, the resulting weights may not be reliable as two different people may not weigh the baby in exactly the same way, or may read the scales differently (Alsop-Shields and Alexander 1997). Measures of reliability are summarised in the table below and, as with consideration of the validity of a study, you would look for evidence that the researcher has considered reliability of the measurement tools used in the methodology section of a research report PN Validity and reliability – What’s it all about?