The Mini-IPIP6: Item Response Theory analysis of a short measure of the big-six factors of personality in New Zealand

T 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 MiniIPIP 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.