BMC Public Health 2004, 4 http://www.biomedcentral.com/1471-2458/4/22 acceptable ways' [1]. Food insecurity may compromise both dietary intakes and health outcomes. The measure- ment of food insecurity in epidemiological studies through the use of self-reported responses to question- naire measures has therefore attracted growing interest. In the United States, an 18 item food security questionnaire was developed [1] and applied in the 1995 Current Popu- lation survey and in subsequent surveys carried out for the US Department of Agriculture [2]. This US food security questionnaire includes items relevant to households with children as well as adults. It has properties analogous to Guttman scaling [3] such that the items can be ranked according to decreasing frequency of affirmative responses and increasing severity of food insecurity. This property permits distinctions to be made between food security, food insecurity without hunger, food insecurity with moderate hunger, and food insecurity with severe hunger [1]. In 1999 Blumberg et al [4] proposed an abbre- viated short form of the household food security scale which included six items from the 18 item questionnaire. These were selected so as to be applicable to households with or without children, and to distinguish between food security, food insecurity without hunger and food insecu- rity with hunger only. This six item module has also been described, with a slightly different question order, by Bickel et al [5]. Application of the food insecurity scale in the US has revealed that food insecurity may be unexpectedly fre- quent even in a high-income country [1]. The concept and measurement of food insecurity may be even more rele- vant in the setting of middle-income countries where a significant proportion of households have incomes which are low enough to potentially compromise access to food. In Trinidad and Tobago, the UN Food and Agriculture Organisation estimated that 13% of the population were under-nourished in 1997–1999 [6]. We recently explored experiences of food insecurity in a population survey in Trinidad and Tobago using the short form of the house- hold food security scale [7]. We found that 25% of sub- jects were classified as food insecure according to this instrument. Food insecurity was associated with low incomes, physical limitations and lower consumption of fruit and vegetables. These findings were consistent with reports from the US [8,9]. However, in contrast to some other studies [10-12], underweight was associated with food insecurity but overweight or obesity were not [7]. While the consistency of these associations with those of other reports pointed to the validity of the scale, our results showed an unexpectedly higher proportion of affirmative responses to the item concerning 'balanced meals' [13], and an unexpected difference in the preva- lence of food insecurity across ethnic groups. The latter finding might have been explained by sampling imbal- ances, as a cluster sampling technique was used [14]. Based on these two observations, Frongillo [13] ques- tioned the validity of the short form food security for application in Trinidad and Tobago. The present report therefore aimed to evaluate item responses obtained using the short form household food security scale in a context which is different from the one in which the measure was developed. Trinidad and Tobago is an English-speaking country but a distinct vernacular is widely used, especially by lower income groups. Food habits have been influenced by the cultures of European, African, Indian and Chinese immi- grants to the country. More recently, restrictions on trade have been dismantled, American economic influences have become increasingly important and there have been associated changes in dietary habits. Based on a compari- son of statistics, the gross national income per capita in Trinidad and Tobago was about 14% that of the US being US$4,930 in 2000 [15]. In order to evaluate the reliability and validity of responses to the short form food security scale, we first explored the consistency of the scale items [3] and esti- mated agreement between responses obtained from indi- vidual respondents from the same households. We then used item response theory models to estimate item thresholds, and to explore whether there was evidence of differential item functioning by ethnicity. In order to eval- uate criterion-related validity [16], we estimated the asso- ciation of each item with household income and frequent consumption of green vegetables and salads. Methods Subjects and questionnaire The sampling design for the study and data collection methods have been described previously [7]. We selected a constituency in North Central Trinidad which was con- sidered to be representative of the socio-economic and ethnic characteristics of the country. Within the constitu- ency, 15 clusters, each of about 20 households were selected. Adults aged ≥ 25 years were enumerated and invited to take part in the survey. A questionnaire was administered by interview, including the six questions described by Blumberg et al [4] (see Table 1). Ethnicity was self-reported according to the categories of the Trini- dad and Tobago national census but were reduced for analysis to the categories 'Afro-Trinidadian', 'Indo-Trini- dadian' and 'mixed' and 'other and not known'. Monthly household income was analysed using five categories reported previously [7]. A limited range of food items were included in a short food frequency questionnaire. We evaluated whether more frequently consumed foods, including green vegetables and salads, were eaten at least 5–6 days per week or not [7]. Descriptive data for the Page 2 of 9 (page number not for citation purposes)