BMC Public Health 2004, 4 http://www.biomedcentral.com/1471-2458/4/22 Table 4: Test for differential item functioning by ethnicity. Mean threshold Adjustment 3 'Balanced meal' 2 'Food last' 5 'Skip meal' 8 'How often' 7 'Eat less' 10 'Hungry' Afro-Trinidadian Indo-Trinidadian Mixed 0.773 1.162 1.133 0.000 0.389 0.360 Adjusted threshold estimates from differential item function model 0.346 (0.081) -0.010 (0.077) 0.032 (0.073) 0.553 (0.075) 0.614 (0.083) 0.471 (0.047) 0.642 (0.081) 0.743 (0.121) 0.630 (0.058) 0.879 (0.113) 0.839 (0.144) 0.980 (0.079) 0.730 (0.062) 0.941 (0.171) 1.095 (0.088) 1.486 (0.112) 1.510 (0.166) 1.428(0.072) Table 5: Associations of food security items with monthly household income. Figures are frequencies (row percent) except where indicated. Item (frequency) Affirmatives (%) with monthly income Slope index of income-related inequality Food secure (327) Food insecure (127) 3 Balanced meal (153) 2 Food last (103) 5 Skip meals (91) 7 Eat less (71) 8 How often (68) 10 Hungry (29) ≤US$133 37 (11) 35 (28) 34 (22) 29 (28) 29 (32) 24 (34) 25 (37) 10 (34) >US$1067 Odds ratio (95% CI)a P value 87 (27) 5 (4) 15 (10) 5 (5) 1 (1) 0 (0) 1 (1) 0 (0) - 8.08 (2.91 to 22.5) 3.77 (1.40 to 10.2) 5.35 (1.80 to 15.9) 17.1 (5.15 to 56.7) 19.9 (4.99 to 79.4) 14.3 (3.49 to 58.5) 20.8 (2.67 to 162.5) <0.001 0.009 0.003 <0.001 <0.001 <0.001 0.004 a relative odds of affirmative response for those at the bottom compared with those at the top of the income scale, adjusted for age, gender, ethnic group and clustering by household Table 3 shows the proportion of affirmative responses to each item by ethnic group after omitting 24 cases where ethnic group was 'not known'. The Afro-Trinidadian eth- nic group gave higher proportions of affirmative responses to all items, while the Indo-Trinidadian group gave the lowest. Item-score correlations were generally consistent between ethnic groups. Table 4 gives the esti- mated item thresholds from the differential item func- tioning model. These were adjusted to allow for differences in the overall proportion with food insecurity between groups. The differential item functioning model gave a better fit to the data than the Rasch model (χ2 92.73, df 10, P < 0.001). There was evidence that the 'bal- anced meal' item gave a relatively higher threshold in the Afro-Trinidadian group than in the other groups. Never- theless, this item was still ranked lowest in terms of rela- tive severity in each ethnic group. There was weaker evidence that the 'eat less' and 'how often' items were ranked differently in the three ethnic groups. The thresh- old for the 'eat less' item was significantly higher for those of mixed ethnicity compared to reference, but the differ- ence was not significant for the Indo-Trinidadian group. Table 5 shows the associations of each item with house- hold income. As the proportion of affirmative responses to the items decreased, and the proposed severity of food insecurity increased, so the proportion of subjects falling into the lowest income category increased and the propor- tion in the highest income category decreased. The slope index of income-related inequality also increased as the proposed severity of each item increased although estimates were imprecise for the low prevalence items. The slope index of inequality can be interpreted as the relative odds of an affirmative response for those at the bottom of the income scale compared with those at the top. Table 6 shows the associations of each item with frequent con- sumption of green vegetables and salads. Those who were food insecure consumed green vegetables and salads less frequently than those who were food secure. This pattern was consistently observed for each item, with some sug- gestion of a graded effect with increasing severity of the Page 5 of 9 (page number not for citation purposes)