The results of trait assessments consist of numbers representing the amount or strength of a certain characteristic which is being measured. So after taking the MBTI(R) tool it may seem natural to assume that the numbers attached to the results represent strength of a preference, that you are an extreme Introvert or Extravert, a strong Thinking type, an off - the - wall Perceiver, more or less of a Sensing type than a co - worker, and so on. Such phrases are very common in online discussion groups. But nothing could be more incorrect!
(Besides not being a trait instrument, the MBTI tool is actually not even a test at all! To learn more about the important distinction between an indicator and a test, see my blog
"Why the MBTI Tool(R) is Not a Test.")
Rather than measuring strength or amount of a preference, the numbers attached to results on the MBTI assessment make up what is known as a preference clarity index. As the MBTI Manual says on p. 121:
Unlike numerical scores on trait instruments that are designed to reflect an "amount" of the trait being measured, the MBTI preference clarity index (pci) is designed to show only how sure the respondent is that she or he prefers one pole of the dichotomy over its opposite.
How does a preference clarity index work? As it is simply stated on pages 7 and 8 of the Manual, "Preference clarity indexes range from 1 to 30 on each of the four dichotomies. The higher the index, the greater the clarity of preference that can be assumed." An index is a measure of how many votes, so to speak, a person who has taken the MBTI tool has given to each side of a dichotomy. A low preference clarity index indicates almost equal votes on each side, showing a slight preference for one side or the other. Slight is one of four preference categories, the others being moderate, clear and very clear.
As stated above, however, it must be remembered that the MBTI tool is not a trait instrument. Thus the only thing that can be inferred from a high preference clarity index is that it represents a higher degree of clarity for that preference than does a low preference clarity index. In other words, it represents how consistently the respondent "...answered the questions in the direction of the preferred pole of a dichotomy (p. 112)."
For example, my type preferences are for INFJ. Therefore my preference clarity indexes show only how consistently I answered the Indicator's questions on the side of Introversion, Intuition, Feeling and Judging. They show how likely it is that my preferences have been correctly reported. As you can see in the above photo, all four of my preferences fall in the clear or very clear categories, so it is very likely that they have been correctly reported.
A high preference clarity index does not necessarily mean one feels more confident or comfortable than do other people, especially those with low preference clarity indexes, in the exercise of that preference. As the Manual says on p. 8, "Thus unlike trait scores, health, competence, or maturity cannot be inferred from the numerical preference clarity index of the MBTI instrument." Confusion results when scores are "...viewed as reflecting traits that identify greater or lesser proficiency, maturity, or accessibility of a preference (p. 112)." It is incorrect to assume that a higher preference clarity index means someone has a preference that is better developed than does someone who has a lower preference clarity index for the same preference.
For example, someone with a preference clarity index of N30 (N stands for Intuition) is not necessarily more Intuitive than someone with a preference clarity index of N15. It just means the person with a higher number is clearer about what he or she prefers, when forced to choose.
It's crucial to remember that clear does not equal more. Like Indicator and Trait Instrument, the animals to which these terms pertain, they are from completely different species. And while preference clarity indexes are certainly interesting, it's more important to know the direction of a preference, which side of a dichotomy the majority of our votes fall on, than whether that preference is slight, very clear or somewhere in between.