The only way we can tell if a scale measures a totally new trait or is
merely some variation on a familiar one is by measuring a group of peo -
ple on the proposed new trait as well as on all the older, known traits
and seeing whether the measures of the new trait really give us somenew information. (The technical name for this process is discriminant
validation .) Since there are hundreds of scales in use, it is virtually im -
possible to measure all of them on an adequate sample of people and so
settle the issue once and for all. But if we piece together all the researchof the past 50 years in which a few traits have been measured jointlyand compared, we can begin to see the outlines of the forest we are in.And once we have some notion of the major groups of personality traits,it becomes much easier to judge the contribution of new candidates. Inshort, we will have begun to map out the territory of personality andobtain the equivalent of the astronomer’s map of the heavens and theentomologist’s taxonomy of insects.
The major tool in this endeavor has been factor analysis, a math-
ematical technique for summarizing the associations among a groupof variables in terms of a few underlying dimensions. The degree towhich two traits go together, or covary , is assessed by the correlationcoefficient, which is fairly easy to interpret. But as the number oftraits increases, the combinations soon surpass the ability of the un-aided mind to comprehend: When 35 trait scales are examined thereare 595 different correlations; 50 scales yield 1,225 correlations. Fac-tor analysis offers a way to determine how to group together sets oftraits that are all related to each other and unrelated to other sets oftraits. For example, anxiety , anger, and depression cluster together aspart of the broader domain of Neuroticism; they are all relatively in -
dependent of traits such as sociability and cheerfulness, which are
part of Extraversion.
It is its ability to condense and organize great quantities of informa -
tion that makes factor analysis a natural instrument for researchers in -
terested in studying the organization and structure of traits. As an ana -
lytic tool, however, factor analysis is not infallible. Different researchers
can arrive at different conclusions from the same data when they usefactor analytic techniques differently . Our view is that factor analysisshould be guided by what we already know about personality structure,just as experiments are guided by theoretically derived hypotheses.Used in this way , factor analysis has led to solid and replicable findingsabout the structure of traits.A Trait Approach to Personality 33