Question

What is, in general terms, the main difference between reliability and validity? Can scales be reliable but not valid? Can scales be valid but not reliable? Please use an example for your explanations.

Answer 1

The terms, validity and reliability are commutable when they are not associated with statistics. When serious statisticians use these terms they derive and talk about high value accurate measurement from different scales. Reliability is just a synonym for consistency. If anyone person records the same personality test repeatedly and always gets same records or results, the test is mean to be reliable.

Validity and Reliability are totally independent of each other. A measurement maybe considered as valid but not reliable, or may be reliable but not valid. The key differences between validity and reliability are:

Validity means that a instrument or a test is accurately measuring what it is made or designed for while reliability refers to the repeated similar results of a particular identity. While considering instruments, a reliable instrument does not mean that it is always valid but a valid instrument is always considered as reliable. Validity is always considered more valuable than reliability when evaluated on a multi item scale. Accessibility of reliable instruments is always easy while handling valid instruments is much tougher. The validity focuses on accuracy for example it checks functionality of scale whether its producing the desired results or not on the other hand reliability focuses on precision and concentrates checking the consistency of the results or readings.      

For the scales to be reliable and valid, It does not only requires to tell you the same weight every time you measure and step on the scale, but it had to measure your actual and correct weight. A test can be reliable by giving consistent readings without being valid but a test cannot be valid unless it provides the accurate readings. A scales is valid only when provides accurate results but can be reliable but giving same repeated results. For example if the weighing scale is reliable it will provide you the same results every time you step on it unless your weight has not changed but the the scale is not working properly it will not give your actual weight, the weight may not be your actual weight so in this example a scale is consistent or reliable but not valid, for scales to be valid it has to measure your actual weight and not display the same weight every time.