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Differential Item Functioning of a Family Affluence Scale: Validation Study on Data from HBSC 2001/02

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Abstract

Methodology for making cross-national comparisons is an area of increasing interest in social and public health related research. When studying socio-economic differences in health outcomes cross-nationally, there are several methodological issues of concern, especially when data is derived from self-reported questionnaires. Health Behaviour in School-aged Children (Currie et al. 1998) is a WHO cross-national study using school samples. HBSC provides comparable data, and thereby a unique opportunity to study associations between social indicators and health outcomes within an international perspective. In 2001/02 data was collected from a total of 162,323 children in 32 countries (Austria, Belgium, Canada, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Greenland, Hungary, Ireland, Israel, Italy, Latvia, Lithuania, Macedonia, Malta, Netherlands, Norway, Poland, Portugal, Russia, Slovenia, Spain, Sweden, Switzerland, Ukraine, United Kingdom, USA.). Studies of social inequalities requires that a comparable measure of socio-economic position (SEP) is in use. HBSC has developed a proxy for social position measuring material wealth, the Family Affluence Scale (FAS). This paper studies FAS and whether it is comparable across population subgroups defined by country, age and gender. Initial analysis revealed that an item measuring perceived family wealth was a valid FAS item. Including this item in the FAS score will improve the reliability of FAS. Graphical log-linear Rasch models (GLLRM) showed that FAS contain differential item functioning (DIF) with respect to country, age, and gender as well as local dependency (LD) between items. During the analysis, test equating techniques where used to adjust for the test bias generated by DIF. We recommend that the equated scores are used whenever FAS is included as a variable. This study suggests that HBSC-FAS should contain five items (additional item: perceived family wealth) when analysing data from HBSC 2001/02, and furthermore that each country should adjust for the DIF or make use of the converted FAS scores provided. If using FAS as a proxy for social position at an international level, it is not advised to compare the absolute levels of FAS, but weigh the scale by ridit transformation.

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Notes

  1. These examples are taken from discussions with colleagues at the Department of Social Medicine.

Abbreviations

HBSC:

Health Behaviour in School-aged Children

SEP:

Socio Economic Position

FAS:

Family Affluence Scale

HASC:

Home Affluence Scale

DIF:

Differential Item Functioning

LD:

Local Dependence

GLLRM:

Graphical Log-Linear Rasch Models

CLR:

Conditional Likelihood Ratio test

PFW:

Perceived Family Wealth

IRT:

Item Response Theory

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Acknowledgements

The HBSC study is an international study carried out in collaboration with WHO Europe. The international coordinator is Professor Candace Currie from the University of Edinburgh, and the international databank manager is Dr. Oddrun Samdal from Bergen University. The 2001/02 survey was conducted by personal responsibility of principal investigators (PI’s) in 35 countries. The authors are grateful for the insightful and constructive comments to earlier drafts from Ms. Kate Levin and Ms. Dorothy Currie from the University of Edinburgh, and also for constructive feedback late in the process from the PI’s.

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Correspondence to C. W. Schnohr.

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Schnohr, C.W., Kreiner, S., Due, E.P. et al. Differential Item Functioning of a Family Affluence Scale: Validation Study on Data from HBSC 2001/02. Soc Indic Res 89, 79–95 (2008). https://doi.org/10.1007/s11205-007-9221-4

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