Are trait-outcome associations caused by scales or particular items? Example analysis of personality facets and BMI.

In personality research, trait-outcome associations are often studied by correlating scale sum-scores with an outcome. For example, an association between the NEO Impulsiveness scale and body mass index (BMI) is often interpreted to pertain to underlying trait Impulsiveness. We propose that this expectation can be corroborated by testing for Spearman’s theorem of indifference of indicator. Namely, an underlying trait-outcome association should not depend on the specific items (i.e., indicators) used to measure the trait. To test this theorem, we outline an indicator exclusion procedure and demonstrate its viability using a simulation design. We then apply this procedure to test personality-BMI associations for indifference of indicator in a large population-based sample of adult Estonians (N=2581) using self- and informant ratings obtained with the NEO Personality Inventory-3. Our results show that the N5: Impulsiveness-BMI association mostly depends on two eating-related items, suggesting that the trait associated with BMI may be narrower than the trait the N5: Impulsiveness scale is supposed to measure. Associations between BMI, E3: Assertiveness and C2: Order seem to pertain to the trait. In sum, testing for indifference of indicator provides a potentially useful method to clarify trait-outcome relationships. R scripts are provided that implement the indicator exclusion procedure.