What is the relationship between reliability and validity?
Reliability and validity are closely related. To better understand this relationship, let’s step out of the world of testing and onto a bathroom scale.
If the scale is reliable it tells you the same weight every time you step on it as long as your weight has not actually changed. However, if the scale is not working properly, this number may not be your actual weight. If that is the case, this is an example of a scale that is reliable, or consistent, but not valid. For the scale to be valid and reliable, not only does it need to tell you the same weight every time you step on the scale, but it also has to measure your actual weight.
Switching back to testing, the situation is essentially the same. A test can be reliable, meaning that the test-takers will get the same score no matter when or where they take it, within reasonably analogous circumstances. But that doesn’t mean that it is valid, or measuring what it is supposed to measure. A test can be reliable without being valid. However, a test cannot be valid unless it is reliable.
Another way to think of it is that a test can give a consistent, poor result. However, it cannot give a good result unless it is consistent.