BMC Public Health 2009, 9:237 http://www.biomedcentral.com/1471-2458/9/237 Variables Outcomes were dichotomously coded and defined as 1) PTB, less than 37 weeks gestation, and 2) SGA birth (birth- weight less than 10th percentile for age and sex for infants from 22 to 43 weeks gestation) [31]. Extreme cases of PTB (N = 50, defined as less than 20 weeks gestation) were excluded as these likely represent implausible values [36]. Gestational age was missing for 64 births; SGA status could not be calculated for 393 births because gestational age was either missing or not within 22 to 43 weeks. One final file for each outcome was created after exclusion of extreme and missing data on PTB and SGA birth (NPTB= 353,006; NSGA = 352,727). Individual-level covariates included maternal education (no high school diploma, high school diploma, some post-secondary, some university or more, or unknown, representing less than 11, 11, 12–13, and 14+ years of education, respectively), birthplace (foreign-born, Cana- dian-born, or unknown), age (less than 20, 20–34, or 35+ years), civil status (married, cohabiting, single, or unknown), previous live births (none, one or more), and sex of infant. These factors have been previously shown to be associated with adverse birth outcomes [4]. Three area-level variables were calculated, using aggre- gated 2001 Canada Census data for household income and immigration status: 1) Median household income, used as an indicator of neighbourhood poverty/economic deprivation; 2) Proportion immigrant population, used as an indicator of immigrant density; 3) Coefficient of varia- tion (CV) of mean household income, used as an indica- tor of income inequality. The CV was calculated according to standard methods, after verification that household income was normally distributed [37]. Large CVs indicate areas with greater inequality. Tertiles of neighbourhood variables were examined. Each tertile contained approxi- mately one-third of births. Statistical analysis Descriptive analyses were performed, and trends were examined using the Cochran-Armitage test. Because the data were hierarchically structured, we used multi-level multivariate logistic regression with CLSC specified as a random effect to model the association between area var- iables and birth outcomes [38]. Multi-level analyses con- sisted of a set of three models, one for the crude association between area variables and birth outcomes, a second adjusting for individual-level covariates (maternal education, birth place, age, civil status, previous births, infant sex), and a third adjusting for individual and all three neighbourhood variables. We estimated odds ratios (OR) and 95% confidence intervals (CI). Statistical signif- icance was determined with the Wald statistic. Models for SGA birth did not adjust for sex of infant because sex is accounted for in the calculation of SGA percentile [31]. Missing data for independent variables were included as separate categories in analyses. To examine the influence of area variables on birth out- comes across social markers potentially serving as effect modifiers, models were re-run after stratifying data according to maternal birthplace (foreign- versus Cana- dian-born) and education (high school diploma or less versus some post-secondary or more). Final models were run a second time using Generalised Estimating Equations (GEE) for correlated observations with a logit link [39]. Both multi-level models and GEE have been used in the literature to evaluate neighbour- hood effects [23,40]. Multi-level models yielded similar ORs and CIs as GEE models, and low intraclass correla- tions (PTB 0.0048, SGA birth 0.0032 – calculated using the latent variable method [41]). Where convergence of multi-level models did not occur due to limited between- cluster variation, we provide results for GEE models. GEE invokes a population-averaged estimate (or marginal mean) not matched on cluster, and overcomes conver- gence problems by estimating the within cluster similarity of residuals as a basis for regression parameters and stand- ard errors [42]. In contrast, multi-level models parameter- ize the mean response conditional on random effects and model the between cluster variation [42]. SAS 9.1 software was used for analyses (SAS Institute Inc, Cary, NC, 2002). Multi-level models were run using the GLIMMIX macro [43], and GEE models using the REPEATED statement of PROC GENMOD. This study was conducted as part of the Québec population health sur- veillance plan mandated by the health ministry and approved by the Public Health Ethics Committee. Results Descriptive characteristics Areas had a mean CV of 18.5% (range 2.3%–47.1%), mean median household income of $41,352 (range $23,891– $79,163), and mean immigrant proportion of 7.7% (range 0%–62.0%). PTB and SGA birth represented 6.3% and 8.3% of births, respectively (Table 1). PTB and SGA birth were progressively prevalent among mothers with lesser levels of education. The frequency of PTB among foreign-born mothers was similar to native-born mothers (6.3%), but SGA birth was slightly more com- mon in foreign-born (9.5%) than native-born (8.1%) mothers. PTB and SGA birth were positively related to area poverty and inversely related to income inequality. While PTB was slightly more frequent in areas with low immi- grant density, SGA birth was more frequent in areas with high immigrant density (Table 1). Page 3 of 11 (page number not for citation purposes)