Jia et al. BMC Cancer 2010, 10:446 http://www.biomedcentral.com/1471-2407/10/446 prior to twelve years of age) and adulthood. The dietary habits involving Chinese herbal plants included the con- sumption of Canton-style herbal tea and herbal slow- cooked soup, both of which involve about 30 herbal plants. Regarding food, intake frequency of salted fish, salted vegetables, preserved/cured meat, fermented pastes and fresh fruits were collected; subjects were asked to choose from five intake frequency categories: never, sometimes, monthly, weekly and daily. Data analysis Statistical analyses were conducted using STATA soft- ware, version 10.0. In the analyses, successive intake fre- quencies were pooled together to eliminate rare categories. Odds ratios (ORs) and corresponding 95% confidence intervals (CIs) were estimated using a logistic regression model, adjusting for age, sex, education, dia- lect and habitation household type. Linear trend tests for exposure-disease associations were performed for continuous variables. Results The demographic characteristics and socioeconomic sta- tus of the study population are shown in Table 1. A total of 1,387 eligible cases and 1,459 healthy controls Table 1 Demographic characteristics and socioeconomic status of the study populations Case (%) Control (%) P-value 1025(74) 362(26) 1038(71) 421(29) Page 3 of 7 724/ 1.0 1075 236/116 3.04(2.38- 3.87) 415/254 2.45(2.03- 2.94) < 0.001 551/962 1.0 204/129 2.72(2.13- 3.48) 624/355 3.17(2.68- 3.77) < 0.001 859/ 1242 55/49 35/27 1.0 1.67(1.09- 2.54) 2.09(1.22- 3.60) 0.001 1056/ 1.0 1203 149/80 2.12(1.59- 2.82) 174/164 1.22(0.97- 1.54) 0.001 1176/ 1.0 797 92/166 0.38(0.28- 0.50) 106/481 0.12(0.09- 0.16) 0.001 Table 2 Association of dietary factors with NPC Intake frequency * Adulthood Childhood Case/ Ctrl OR(95%CI)# Case/ Ctrl OR (95% CI)# Salted fish Less than monthly 1085/ 1.0 (ref) 1272 Monthly 162/80 2.39(1.80- 3.17) Weekly or more 128/98 1.58(1.20- 2.09) P trend < 0.001 Salted vegetables Less than monthly 970/ 1.0 (ref) 1252 Monthly 218/85 3.31(2.54- 4.31) Weekly or more 190/113 2.20(1.71- 2.82) P trend < 0.001 Preserved/cured meat Less than monthly 606/ 1.0 (ref) 1256 Monthly 170/46 7.80(5.33- 11.39) Weekly or more 157/37 9.69(6.39- 14.70) P trend < 0.001 Fermented pastes Less than monthly 1243/ 1.0 (ref) 1301 Monthly 76/44 1.82(1.24- 2.66) Weekly or more 56/106 0.57(0.41- 0.79) P trend 0.157 Fresh fruits Less than monthly 299/234 1.0 (ref) Monthly 138/91 1.22(0.85- 1.76) Weekly or more 936/ 0.63(0.51- 1126 0.77) 0.157 Variables Sex Male Female Age Mean (SD) < 30 30-40 41-50 51-60 > 60 Dialect Hakka Cantonese Hokkien Others Education None or primary school Secondary school High school University or more Household type Rural Urban 46.92 11.34 84(6) 323(23) 442(32) 371(27) 167(12) 219(16) 917(66) 133(10) 116(8) 282(20) 373(27) 459(33) 269(20) 965(70) 412(30) 47.34 11.64 78(5) 352(24) 431(30) 414(28) 184(13) 206(14) 971(67) 146(10) 132(9) 297(20) 391(27) 479(33) 286(20) 1018(70) 429(30) 0.100 0.564 0.603 0.998 0.874 P trend * Intake frequency was categorised as follows: less than monthly, < once per month; monthly, once per month to once per week; weekly or more, ≥ once per week. # OR, odds ratio calculated among matched case and control groups, adjusting for sex, age, education, dialect and household type. were recruited and used in the analyses. As expected due to our matching strategy, the distribution by age, sex, education, dialect and rural/urban resident type were comparable between cases and controls. All of the subjects were of Han ethnicity. Table 2 presents the relationship between dietary fac- tors and NPC risk in the case-control dataset. Con- sumption of salted fish, salted vegetables and preserved/