Academic journal article New Zealand Journal of Psychology

Measuring Psychological Distress in New Zealand: Item Response Properties and Demographic Differences in the Kessler-6 Screening Measure

Academic journal article New Zealand Journal of Psychology

Measuring Psychological Distress in New Zealand: Item Response Properties and Demographic Differences in the Kessler-6 Screening Measure

Article excerpt

From the early 1980s, the standard practice for measuring psychopathology in a population was through fully structured diagnostic interviews conducted in epidemiological surveys (Kessler et al., 2002). With the use of such interviews, community and nationally representative surveys have provided important prevalence rates for the number of people in the general population who meet the Diagnostic and Statistical Manual's (fourth edition; DSM-IV) criteria for having one or more psychiatric illnesses in their lifetime, while also highlighting the general prevalence rates of psychological distress at any given point in time (Kessler et al., 2002). However, the use of fully structured diagnostic interviews in nationally representative samples is highly complex and time consuming (Pinninti, Madison, Musser, & Rissmiller, 2003; Oakley Browne, Wells, Scott, & McGee, 2010; Mitchell & Beals, 2011). Consequently, short screening scales that are cheaper to administer, and less burdensome for participants, have been developed to complement these fully structured diagnostic interviews.

Here, we assess the item response properties for the Kessler-6 (K6), a short screening measure of nonspecific psychological distress, in a large New Zealand sample. We compare demographic differences in psychological distress using indicators based on item response-weighted K6 scores and classical summative item scores, and discuss the utility of these different scoring methods for the assessment of psychological distress in New Zealand. Finally, we present an in-depth analysis of ethnic group (and other demographic-based) differences in psychological distress.

The Kessler scales, which consist of 6-item and 10-item scales, have been successfully used in a range of population and community surveys around the world (Kessler, Green, Gruber, Sampson, Bromet, Cuitan, et al., 2010; Sunderland, Slade, Stewart, & Andrews, 2011). Here in New Zealand, the Kessler scales have been introduced into the 2006/07 and 2011/12 New Zealand Health Survey (NZHS; Ministry of Health, 2007, 2012) and the 2003/04 New Zealand Mental Health Survey (NZMHS; Mental Health Commission, 2011). We aim to complement these earlier analyses using data from another large--and independent-- national sample conducted in New Zealand: The New Zealand Attitudes and Values Study (NZAVS). Using NZAVS data, we examine the item response properties of the K6. Building on the original analyses of North American data by Kessler et al. (2002), we employ Item Response Theory (IRT) to examine the scale's ability to differentiate between people with low/no psychological distress versus people with mild psychological distress, or between people with mild/ moderate distress versus more extreme levels of distress in the unique context of New Zealand.

We also provide up-to-date population norms for rates of nonspecific psychological distress in New Zealand, with particular focus on differences in prevalence rates between ethnic groups. New Zealand has a highly diverse ethnic population (Sibley & Ward, in press). According to the 2006 census, New Zealanders of European descent compose 67% of the population, Maori 15%, Asians 9% and Pacific peoples 7% (Statistics New Zealand, 2006). Moreover, differences in mental health and psychological distress among Maori, Pacific and European peoples are well documented (e.g., see Harris et al., 2012). We present additional data from 2010 that contribute to this research corpus on demographic differences in mental health.

The Kessler scales

The development of the Kessler scales was based on a review of psychopathological screening scales by Dohrenwend, Shrout, Egri, and Mendelsohn (1980). The scales constitute the first population health screening tools developed using modern IRT (Kessler et al., 2002). Kessler and colleagues (2002) used IRT to select items for their scales which had maximum precision in the clinical range of the latent trait (9) for non-specific psychological distress. …

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