Academic journal article New Zealand Journal of Psychology

The Mini-IPIP6: Item Response Theory Analysis of a Short Measure of the Big-Six Factors of Personality in New Zealand

Academic journal article New Zealand Journal of Psychology

The Mini-IPIP6: Item Response Theory Analysis of a Short Measure of the Big-Six Factors of Personality in New Zealand

Article excerpt

There are a vast number of self-report personality measures available. Generally speaking, these measures are based on the assumption that "... individuals are characterized by stable, distinctive, and highly meaningful patterns of variability in their actions, thoughts, and feelings across different types of situations. These if ... then ... situation-behavior relationships provide a kind of 'behavioral signature of personality' that identifies the individual and maps on to the impressions formed by observers about what they are like" (Mischel, 2004, p. 8). This quote provides a good working definition of personality. The aim of the many available personality measures should be then to provide a method for measuring individual differences in these distinct and highly meaningful patterns of variation, differences in other words, across people in their personality traits.

This study is the second in a series of manuscripts validating a short-form six-factor self-report measure of the six major dimensions of personality for use in the New Zealand context. This measure is known as the MiniIPIP6 (Sibley et al., 2011). The scale extends the previous five-factor Mini-IPIP inventory initially developed by Donnellan, Oswald, Baird, and Lucas (2006). In this paper I present an analysis of the item discrimination and difficulty parameters for the Mini-IPIP6 using Item Response Theory (Samejima, 1969). As I outline below, unlike classical psychometric assessment, Item Response Theory examines the extent to which a set of items provide well-distributed measurement precision across different levels of the latent trait they measure. This study provides, for the first time, a detailed assessment of the response parameters for a public domain short-form measure of personality validated for use in New Zealand. To do so I analyse Mini-IPIP6 scores from the first wave of the New Zealand Attitudes and Values Study. This is a nationally representative longitudinal study of around 6000 New Zealanders.

What is Personality?

Previous research has typically identified five distinct factors, or broad clusters of related traits and behavioural tendencies, which constitute distinct latent dimensions of personality. These five broad-bandwidth dimensions of personality were synthesized and organized into a general framework by Goldberg (1981) who coined the term 'Big-Five' (see also Goldberg, 1990). This Big-Five model of personality contains the following factors: (1) Extraversion, (2) Agreeableness, (3) Conscientiousness, (4) Neuroticism, and (5) Openness to Experience. More recently, Ashton and Lee (2001, 2007, 2009) have made a compelling argument for an extended 'Big-Six' model of personality which adds an additional dimension to the mix: (6) Honesty-Humility. A descriptive summary of the core content and example traits for these different dimensions of personality is presented in Table 1.

Following from the general definition of personality by Mischel (2004) with which I began this manuscript; these six dimensions of personality reflect six distinct and 'highly meaningful patterns of variability in people's actions, thoughts, and feelings.' Why these six dimensions specifically? Evolutionary theory suggests that what we refer to as personality should reflect variation in behavioural systems or ways of acting that were equally adaptive for our species in different ecological niches (MacDonald, 1995, 1998; Nettle, 2006). Personality should reflect those traits in our species where it was sometimes the individuals high in the trait that had an adaptive advantage, but equally often in other situations, it was individuals low in the trait that had an adaptive advantage. Overall therefore, the traits had balanced selection pressures and this resulted in species-wide variation being maintained (Penke, Denissen, & Miller, 2007).

When we talk about personality, this is what we should be aiming to measure: traits which vary across people because such species-wide variation itself is the feature that has been selected for in evolution (Buss, 1991; Denissen & Penke, 2008). …

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