Are personality trait items reliably more or less 'difficult'? : Mokken scaling of the NEO-FFI

The psychometric properties of self-report scales, such as the NEO-Five Factor Inventory (NEO-FFI), are examined using a range of statistical methods such as factor analysis. These methods establish, among other things, whether items in a scale covary and whether this indicates latent traits to the construct under investigation. The present study was designed to apply an additional analytical tool to the NEO-FFI, to investigate whether or not the items in its scales form hierarchies. Mokken scaling is designed to search multivariate data for ordinal, unidimensional and hierarchical scales. It was applied in the present study using the Mokken Scaling Procedure software. NEO-FFI data from 1028 subjects were entered into the Mokken Scaling Procedure. Following checks for scalability, monotonicity and double monotonicity one scale was obtained from the neuroticism dimension of the NEO-FFI. The data indicate that some items related to neuroticism in the NEO-FFI form a reliable hierarchy. There may be practical applications of hierarchical scales, with items having different level of 'difficulty' in personality research.