Measurement Scales

Dr. Bob Kizlik

If you have read the information on the ADPRIMA page on measurement, assessment and evaluation in education, and arrived at this page, you are in the right place.  This page was added several years after the aforementioned page was included on my site.  The reason for this page is to provide you with a little bit of clarification and an expansion of the understandings I hope you gleaned from the page on measurement, assessment and evaluation.

Basically, any time a measurement is made we are looking at an object or condition that meets certain criteria for inclusion or exclusion from descriptive sets. These sets are what I refer to as measurement scales. So if you will indulge me, I will provide a description of these scales so that your understanding of what they mean and how they are used in statistical analysis may be improved.

There are four basic measurement scales. From least complex to most complex, they are: nominal, ordinal, interval, and ratio.  They are fundamental to the process of measurement, and without an understanding of their differences, at best poor information will be derived, and at worse, erroneous conclusions will be reached.

The measurement scale descriptions:

Nominal measurement scales refer to those measurements when the only meaningful  results are the delineations that one thing is different from another. For example, if you have a bag of apples and a bucket of coal, the only measurement possible involves the nominal scale. All you can say is that one set is apples and the other set is coal. It is a measurement where the only conclusion you can reach is that one thing is different from another. Another way to consider the nominal measurement scale is to think of it as a basic classification system. It might also be worthwhile to take a look at the behavioral verb "classify."  In the nominal scale you are essentially classifying by name. It is always a good idea to be as clear as possible when doing this.

Ordinal measurement scales refer to those measurements where the results indicate only that one thing is either greater or lesser than another. This always means a measurement that explicitly implies that the objects, events or processes and be placed into some order. The assigning of grades based on scores is an example of this scale, with, for example, the observation that a grade of "A" represents not only a different value than a grade of "C" but that it also represents a higher or greater value.

Interval measurement scales refer to those measurements where there are equal intervals between given values. Interval scales are used in almost every aspect of common measurement. A ruler employs an interval scale. That means that the distance between three inches and six inches is the same as the distance between nine inches and twelve inches. In a room thermometer, the difference in degrees between 72 Fahrenheit and 78 Fahrenheit is the same as that between 90 degrees Fahrenheit and  96 degrees. The intervals are the same.

Ratio measurement scales are the same as ordinal scales with one important difference. The difference is that ratio measurement scales contain a zero.  the inclusion of a zero allows for negative values to be expressed in relation to a positive value. The most obvious and easily understood example of a ratio measurement scale is an outdoor thermometer. The intervals are equal, but whether Fahrenheit or Celsius, measurement values can be expressed as a negative, as in -10 degree Celsius.

So there in a nutshell you have it. Measurement always involves some sort of scale, and the observations linked to the measurements can be noted as a simple difference of name and thus a simple classification. One step up in complexity is the ordinal scale which implies the there is an order to the object or process, and one thing can be said to be not just different, but greater or lesser than another. The next up in cmplexity, the interval scale is the most frequently used for measurement and rests on the certainty of equal intervals between sequential points on the scale. Finally  there are ratio scales, which are exactly like interval scales with the addition of a zero point.

This is provided to give a little perspective on the description of educational measurement.

Click here to return to the page on educational measurement, assessment and evaluation.