The categories are ordered and have unit properties. A scale with five categories, e.g. , is more precise than a scale with two categories. The precision of a scale can collapsed to a lower level. A good instrument is both reliable and valid . A bad instrument is either unreliable, invalid, or both .
Reliability refers to the reproducibility of a measurement.
A reliable instrument yields roughly similar measurements at different times, in different places, and when used by different researchers. An unreliable instrument yields very dissimilar measurements over time, place, and researcher. This occurs because, as the number of categories increases, the chance of a misclassification error increases. There are limits to this method, of course. Second, as the number of categories is reduced, the similarity of elements within a category and the dissimilarity of elements in different categories both decrease.