BMC Public Health 2008, 8:149 http://www.biomedcentral.com/1471-2458/8/149 comes recorded between 1998 and 2001/02, while the present study was undertaken after the introduction of retention targets. If drug use is considered a chronic con- dition defined by relapse and re-presentation [18,36], the present finding that a high proportion of both criminal justice and non-criminal justice referrals adhere to treat- ment for at least 12 weeks reflects positively on current policy and practice. While this study investigated the effects of a variety of var- iables, the goodness of fit test suggests that other impor- tant covariates were missing from the model. This finding hints at the complexity of factors associated with treat- ment outcomes and confirms suggestions that drop out and retention is an index that capture the impact of many interrelated individual and process measures [7]. In par- ticular, it was not possible to evaluate the effect of treat- ment modality on treatment retention because an individual's entire treatment episode could be comprised of treatment within different modalities, yet treatment modality is likely to have considerable bearing on treat- ment outcomes. An examination of the covariate patterns of the goodness of fit test showed discrepancies between the observed and expected values for deprivation and fur- ther work looking at the interaction between deprivation and other explanatory variables is needed. However, the reliability of the findings presented here is enhanced by treatment outcome data being unavailable for only 272 (1.6%) individuals. Large-scale prospective cohort studies of drug treatment outcomes are costly and time consum- ing and the robustness of findings are often questionable due to follow-up losses occurring differentially across comparison groups. The use of well-established monitor- ing systems provides a cost-effective means by which treat- ment outcomes can be assessed [18] and while case control studies are also prone to selection bias (with con- trols not adequately reflecting the exposure history of cases), here controls would have been cases if they had dropped out of treatment before 12 weeks. Recall bias, an accepted weakness of case control studies, did not affect results of this study because data were collected prospec- tively; exposure data recorded before treatment outcomes were ascertained. Problems associated with the use of routine data (for example, inconsistencies in the manner in which informa- tion is recorded) are somewhat mitigated in this study by using nationally agreed definitions and reporting proto- cols, standardised coding framework and data validation checks [12]. Furthermore, the large sample size reduces the impact of slight non-systematic variations, increases the study power and reduces the likelihood of a Type II error. However, the study relies totally on self-reported data and the accuracy of such reports cannot be ascer- tained. The standardisation of the drug treatment and data reporting across England mean that the results from the North West of England can be generalised to other areas although results also show that the effect of factors specific to each region would require further considera- tion. Conclusion During the 2005/06 reporting period, 75% of clients were retained in drug treatment in the North West of England. Asian drug users were more likely to drop out of treatment than their white equivalents, and further work into why this is the case is needed in light of evidence to suggest the drug profiles of Asian drug users are similar to those of their white counterparts. Residents of Cumbria and Lanca- shire and Greater Manchester were more likely to drop out of treatment than people living in Cheshire and Mersey- side, and work on the impact of treatment processes, treat- ment quality and aspects of client satisfaction would be welcomed. While overall, those living in the least deprived areas were more likely to drop out than those liv- ing in the most deprived areas, the effect of deprivation varied significantly according to the age of the individual. The effect of deprivation on treatment outcomes requires further consideration, preferably using an individual, rather than geographically aggregated measure of depriva- tion. These findings show that personal characteristics, process measures and situational factors all effect treat- ment outcomes and highlight the complexity of factors associated with treatment success. The large sample size, small proportion of missing data and the inclusion of data quality and standardisation measures increase the validity and reliability of these findings. Finally, the NTA are cur- rently revising the retention target, which is due to change for the 2008/09 reporting cycle. To date, it is not clear how the retention target will change, or whether 12 weeks will continue to be the retention benchmark. Competing interests The authors declare that they have no competing interests. Authors' contributions CMB conceived of the study and the design, conducted the data manipulation and statistical analysis and partici- pated in writing the manuscript. AMM assisted with statis- tical analysis and participated in writing the manuscript. AJEM carried out the data extraction, helped identify back- ground information and helped to write the manuscript. All authors read and approved the final manuscript. Acknowledgements No specific funding was received for this research project. Authors would like to thank all those whose data have been used and the staff who work in treatment services who collect these data. Thanks are also extended to NDTMS staff at the Centre for Public Health, Liverpool John Moores Uni- versity and staff at the North West Public Health Observatory. In addition, the authors would like to thank Dr Stephen Sharp for his advice on the sta- Page 10 of 11 (page number not for citation purposes)