Health Education Research Advance Access originally published online on October 23, 2006
Health Education Research 2006 21(Supplement 1):i98-i109; doi:10.1093/her/cyl087
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© 2006 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Does participation in an intervention affect responses on self-report questionnaires?
1 Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030, USA
2 College of Education, University of California Berkeley, Berkeley, California 94720, USA
3 University of British Columbia, Department of Pediatrics, Centre for Community Child Health Research, L408-4480 Oak Street, Vancouver, BC V6H 3V4, Canada
* Correspondence to: T. Baranowski. E-mail: tbaranow{at}bcm.tmc.edu
There has been some concern that participation in an intervention and exposure to a measurement instrument can change participants' interpretation of the items on a self-report questionnaire thereby distorting subsequent responses and biasing results. Differential item functioning (DIF) analysis using item response modeling can ascertain possible differences in item interpretation by testing for differences in item location between groups. The DIF for treatment versus control group differences at post-intervention assessment and the Time 1 and Time 2 differences in a control group were analyzed using data from a dietary change intervention trial for Boy Scouts. The measures included fruit and vegetable (FV) frequency of consumption, preferences and self-efficacy. Treatment–control group DIF at post-intervention assessment was detected in a higher percentage of items for FV frequency than for preference or self-efficacy. Time 1 to Time 2 differences in items for the control group were detected in one item for each of the three scales. Further research will need to clarify whether the obtained DIFs reflected true changes in frequency, preference or self-efficacy or some reinterpretation of items by participants following an intervention or merely after previous exposure to the measure.
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