Department of Public Health Sciences, Kings College London, 42 Weston St, London SE1 3QD, UK and Ministry of Health, Trinidad and Tobago. E-mail: martin.gulliford{at}kcl.ac.uk
In his commentary on our recent paper,1 Dr Edward A Frongillo2 criticizes our use of a well-described household food security scale in Trinidad. There will always be some uncertainty concerning the application of a given measure as there is no perfect instrument to evaluate food security or dietary patterns in any population. The household food security measure was used in the US national Current Population Survey (which provided the comparison data used in Dr Frongillos commentary) but the application of the instrument to all groups in the multilingual, culturally diverse US population has not been examined sufficiently (ref. 3, p. 8). Questionnaire evaluation must be considered when differences in literacy, language, dialect, or culture, as well as socioeconomic status, may influence responses and this consideration might suggest that an instrument should be tailored to local requirements. It is advisable, however, to be judicious in modifying such measures so as not to compromise the validity or comparability of an instrument. Departures from a previously tested template should only be undertaken to guarantee enhanced performance of a measure. Dr Frongillos comments appear to underestimate both the weight of evidence required to justify an alteration to an established measure and the limitations of local validation studies. Before concluding that a measure gives unsatisfactory results in a given local population, or a particular group within a population, it is essential to ensure that the findings cannot be ascribed to error or bias. There is a relatively high risk that local questionnaire evaluation studies, implemented within the short time scales suggested, will lead to erroneous conclusions if sample sizes are too small or if subjects are insufficiently representative.
While our data suggested an unexpected difference in the frequency of food insecurity according to ethnicity, it would be premature to conclude that the instrument had differential validity in these groups. We had no prior hypothesis about ethnic differences in food insecurity. The study used cluster sampling with the selection of a relatively small number of neighbourhoods. Food insecurity, income, and ethnicity each showed evidence of clustering within neighbourhoods. Imbalances in the characteristics of different groups could arise through chance. A larger study will be required to determine whether this finding will be replicated. Dr Frongillo observes that the ranking of the prevalence of affirmative responses to the first two items differs in our data as compared with the US data. This seems to overemphasize the Guttman-like properties of the scale, since it is not clear that an inability to afford balanced meals should always indicate a greater severity of food insecurity than that for a person finding that her food did not last and being unable to buy more. We agree that the balanced meal item may be unsatisfactory but rather than concluding that this requires the adaptation of the household food security scale in each local setting, special consideration should be given to reviewing this item when the instrument is revised.
A potential for misclassification of food insecurity status does raise a concern that a possible true association between food insecurity and obesity might be attenuated. In order to explore this possibility, we repeated our previous analyses using the same methods but with two modifications to the classification of food insecurity. We first used a cut-point of three rather than two to identify subjects who were food insecure. We then omitted the balanced meal item from the assessment of food insecurity, using a cut-point of two items out of five. The results are shown in the Table. As expected, the estimated prevalence of food insecurity was somewhat lower when the more restrictive definitions were used. The associations of food insecurity with overweight (body mass index [BMI]
25 kg/m2) or obesity (BMI
30 kg/m2) were similar using each of the three definitions of food insecurity. Thus the test of our primary hypothesis concerning food insecurity and obesity was robust to varying the definition of food insecurity. The association of underweight (BMI < 20 kg/m2) with food insecurity was somewhat sensitive to the definition of food insecurity, but this analysis was based on only 41 cases who were underweight. This again points to the need for a larger study.
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References
1 Gulliford MC, Mahabir D, Rocke B. Food insecurity, food choices, and body mass index in adults: nutrition transition in Trinidad and Tobago. Int J Epidemiol 2003;32:50816.
2 Frongillo EA. Commentary: Assessing food insecurity in Trinidad and Tobago. Int J Epidemiol 2003;32:51617.
3 Wolfe WS, Frongillo EA. Building Household Food Security Measurement Tools from the Ground Up. Washington, DC: Food and Nutrition Technical Assistance Project. Academy for Educational Development, 2000. http://www.fantaproject.org/downloads/pdfs/hfs_measure.pdf Accessed 3 September 2003.
4 Gulliford MC, Mahabir D. Relationship of health-related quality of life to symptom severity in diabetes mellitus; a study in Trinidad and Tobago. J Clin Epidemiol 1999;52:77380.[CrossRef][ISI][Medline]