BMC Public Health 2008, 8:149 http://www.biomedcentral.com/1471-2458/8/149 Table 1: Description of the variables included in the final dataset Variable Attributor code (initials, date of birth, sex and D[A]AT of residence) Sex Age at triage Date of triage and discharge Outcome code Ethnicity code D(A)AT of residence Referral source Recent treatment contact Opiate use Stimulant use Alcohol use Deprivation Description Used to identify an individual Female or male Categorised into 18–24, 25–34, 35–44, 45–54 and 55–74 Used to identify the start and end of each episode of treatment Retained, dropped out or other (see results section for other outcomes) White (white British, white Irish, white other) Mixed (white and black Caribbean, white and black African, white and Asian, other mixed) Asian/Asian British (Indian, Pakistani, Bangladeshi, other Asian) Black/Black British (Caribbean, African, other black) Other (Chinese, other) Recoded to public health zone of residence Cheshire and Merseyside (Cheshire, Halton, Knowsley, Liverpool, Sefton, St Helens, Warrington, Wirral) Cumbria and Lancashire (Blackburn, Blackpool, Cumbria, Lancashire) Greater Manchester (Bolton, Bury, Manchester, Oldham, Rochdale, Salford, Stockport, Tameside, Trafford, Wigan) Recoded to criminal justice and non-criminal justice referral route Non-criminal justice referral route (Accident and Emergency departments, Community Care Assessment, Connexions, drug service: non-statutory, drug service: statutory, Education Services, Employment Services, General Practitioner, looked after children, Psychiatry, self, Social Services, needle exchange services) Criminal justice referral route (Arrest Referral/Drug Interventions Programme, CARAT, Drug Treatment and Testing Order, Probation, Youth Offending Team) No (no treatment contact in 2003/04 or 2004/05) or yes (treatment contact in 2003/04 or 2004/05) No or yes (heroin, methadone, other opiates) No or yes (amphetamine, cocaine, crack cocaine) No or yes Assessed using a composite score: the Index of Multiple Deprivation. The index is based on data for seven domains: income, employment, health and disability, education, skills and training, barriers to housing and services, living environment and crime. A higher score reflects a higher level of deprivation. Table 2 shows the scores for each D(A)AT area and the quintiles used for the analyses. The LRT was used for three analyses. 1) to test the hypothesis that the variable under consideration was not significantly associated with drop out; 2) to compare a model with a linear trend with a model with individual effects across strata for ordinal explanatory variables (age group and deprivation quintile) and 3) to compare a model with and without an interaction term between age group and, separately, each other variable. A LRT statistic of P < 0.05 indicated significant differences between the two models being compared. For model simplicity, com- mon odds ratios were used where a linear trend was iden- tified and interaction terms were not included in the model if the interaction term did not significantly improve the model's fit. Additive models were not consid- ered because odds were being modelled. Pearson's chi- square was used to evaluate the fit of the model to the data. All analyses were undertaken using STATA inter- cooled version 8 [19]. The Ethics Committee of the London School of Hygiene and Tropical Medicine granted ethical approval. Results In 2005/06, 22330 triage assessments were recorded by the NDTMS database for the North West of England for drug users aged between 18 and 74. This figure included multiple triage assessments for the same individual when they had been seen by two different treatment services or had started a new period of treatment at the same service on multiple occasions. Following data preparation in the manner detailed above, 16626 individuals aged between 18 and 74 began a new episode of treatment in the North West of England in 2005/06. Data on treatment outcomes were unavailable for 272 (1.6%) individuals. Of the remaining 16354 people, 12230 (74.8%) were retained in treatment for 12 weeks or more and 1649 (10.1%) dropped out of treatment before 12 weeks. A further 2475 (15.1%) episodes resulted in other treatment outcomes. These other treatment episode outcomes were as follows: treatment completed in less than 12 weeks, client drug free (n = 275, 1.7%), treatment completed in less than 12 weeks, client not drug free (n = 326, 2.0%), treatment withdrawn, contract breached by client (n = 379, 2.3%), no appropriate treatment available (n = 106, 0.6%), treat- ment declined by client (n = 1, <0.1%), inappropriate Page 4 of 11 (page number not for citation purposes)