BMC Public Health 2004, 4 http://www.biomedcentral.com/1471-2458/4/22 reported for the food security measure in the US [21,22]. Furthermore the classification of food insecurity provided information additional to that from socio-economic sta- tus in terms of income and education. In the US data, the item concerning 'food not lasting' gave the highest proportion of affirmatives [1]. In our data, the 'balanced meal' item represented the item with lowest severity but the ranking obtained for the 'balanced meal' item was also reflected in the estimated associations with income and consumption of vegetables and salads. The findings from the Trinidad and Tobago data are therefore consistent even though they differ in this respect from the US data. However, there was evidence that the 'balanced meal' item performed differently in different ethnic groups, with a higher threshold being observed in the Afro-Trinidadian group which nevertheless exhibited the highest proportion with food insecurity in this sample. This differential functioning was not sufficiently large to influence the ranking of the item in the different groups. The 'eat less' and 'how often' items generally had similar thresholds while the 'hungry' item gave a much higher threshold. Therefore it is justifiable to conclude that the different relative item thresholds compared with US data, does not require alteration to the cut-points of two affirmatives for the classification of 'food insecurity with- out hunger' or five affirmatives for 'food insecurity with hunger'. Qualitative data from Hawaii showed that the 'balanced meal' item may be subject to variable interpretation [23] and this was reflected in the performance of the food secu- rity measure [24]. Harrison et al [25] also questioned the conceptual validity of this item in the translation of the food security questionnaire into Spanish. We share the reservations expressed by Derrickson et al [23] concerning this item. Derrickson et al [23] suggested that additional explanation of this item should be provided. In a more recent survey we have introduced the following text: 'a balanced meal may contain starchy food, like rice, pota- toes, bread, ground provisions or macaroni ; and a pro- tein-rich food like meat, fish, milk, or peas or beans; and a fruit or a vegetable'. This was adapted from Derrickson et al based on the advice of experienced local nutritionists. However, the requirement for additional explanation might support the idea that the original question is unsat- isfactory. Qualitative research might be carried out to identify a more culturally appropriate wording for this item but changes to an established measure should gener- ally be made cautiously because changes made locally may compromise the generalisability of findings. Our study had the strengths of a population-based study based on a multi-stage sampling design. The question- naire was interview administered and there were few miss- ing values. The sample size was relatively modest but adequate for this purpose. A concern relates to the relative representativeness of the samples obtained for each ethnic group. The sample was drawn from 15 clusters of house- holds and income, ethnicity and food insecurity were each correlated within clusters. Imbalance in the distribu- tion of income, or food security, in relation to ethnicity could have occurred by chance through the sampling of a small number of clusters. Post hoc sub-group analyses by ethnicity therefore require replication and we caution against attaching undue weight to these findings, espe- cially since there were few affirmative responses for some items when analysed by ethnicity. In the analyses, we were not always able to allow for the effects of clustering by household or household cluster because clustered ver- sions of relevant tests were not available. In general the consequence would be to make standard errors and P-val- ues too small, the results should therefore be viewed with some caution. A considerable research effort, spanning almost a decade, was devoted to the development of the US food security measure [1,21,22]. The items were derived from the results of qualitative interviews with food insecure sub- jects, and the instrument is considered to be well grounded. Wolfe and Frongillo observed that the compa- rability of the instrument is likely to hold in 'many other countries' [26]. The present results suggest that the instru- ment may certainly prove of value in assessing food inse- curity within countries like Trinidad and Tobago which may have a higher frequency of food insecurity than the US. Our results generally support the reliability and valid- ity of the food security scale within this population but, in agreement with other reports, we find that the 'balanced meal' item gave less consistent responses. Like any brief instrument, the short form household food security scale is unlikely to encompass all facets of the experience of food insecurity. There is a role for future research to eval- uate whether relevant aspects of food insecurity are neglected by the instrument, and whether the sensitivity and specificity of the resulting classification can be enhanced. Nevertheless, the present results support the use of this measure in epidemiological studies to evaluate the distribution, determinants and associations of food insecurity within this population. As with other self-reported measures, there are likely to be difficulties in making comparisons across different popu- lations because individuals' expectations and their inter- pretations of item response categories may vary systematically between populations [27]. This is evident in the results obtained for the 'balanced meal' item. The difference in prevalence of food insecurity between Trini- dad and Tobago and the US is large and consistent. Nev- ertheless, comparisons between populations of the Page 7 of 9 (page number not for citation purposes)