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Issues in assessing the validity of nutrient data obtained from a food-frequency questionnaire: folate and vitamin B12 examples

Published online by Cambridge University Press:  02 January 2007

Victoria M Flood ,
Wayne T Smith ,
Karen L Webb  and
Paul Mitchell
Victoria M Flood
Affiliation:
Department of Public Health, University of Sydney, Sydney, Australia
Wayne T Smith
Affiliation:
Centre for Clinical Epidemiology and Biostatistics, Newcastle University, Newcastle, Australia
Karen L Webb
Affiliation:
Department of Public Health, University of Sydney, Sydney, Australia Department of Molecular and Microbial Biosciences, University of Sydney, Sydney, Australia
Paul Mitchell*
Affiliation:
Department of Ophthalmology (Centre for Vision Research), University of Sydney, Sydney, Australia Westmead Millennium Institute, Centre for Vision Research, Westmead Hospital, Westmead, New South Wales 2145, Australia
*
*Corresponding author: Email paul_mitchell@wmi.usyd.edu.au
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Abstract

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Objective:

To compare methods used to assess the validity of nutrient intake data obtained from a food-frequency questionnaire (FFQ), using folate and vitamin B12 as nutrient examples.

Design:

Cross-sectional sample from a population cohort.

Setting:

Two postcode areas west of Sydney, Australia.

Subjects:

In total, 2895 people aged 49 years and older provided dietary data using a semi-quantitative FFQ (79% of 3654 subjects examined). The validity of the FFQ was assessed against three 4-day weighed food records (WFRs) completed by 78 people (mean age 70 years).

Results:

Folate and vitamin B12 validity data were assessed using different methods. The Spearman ranked correlations (energy-adjusted) were 0.66 for folate and 0.38 for vitamin B12. Using the Bland–Altman method, following loge transformation, no linear trend existed between the differences and means for folate and vitamin B12. Large differences existed between the FFQ and WFR in individual cases, particularly for vitamin B12. Finally, data were divided into quintile categories for the test and reference method: 79% classified folate within one quintile, 65% classified vitamin B12 within one quintile; there was no gross misclassification for folate and only 3% misclassification for vitamin B12.

Conclusions:

Different methods of analysis provided different information about the validity of the FFQ. Correlation coefficients should not be used alone to assess the validity of nutrient data, but should be used in conjunction with Bland–Altman analyses. Depending on the use of the data, additional assessment of classification categories is recommended. This worked example demonstrates that absolute intakes of folate and vitamin B12 should be used with caution.

Keywords

ValidityFood-frequency questionnaireFolateVitamin B12Blue Mountains Eye Study
Type
Research Article
Information
Public Health Nutrition , Volume 7 , Issue 6 , September 2004 , pp. 751 - 756
Copyright
Copyright © CAB International 2004

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