Int. J. Environ. Res. Public Health 2012, 9 194 5. Duration of observation is likely to be an issue. Many smokers have given up for a day; few give up for a month, and only a small percentage for a year or more. Comparing studies of different duration may be problematical. 6. Issues around size. Many studies in addiction are quite small, with a few tens of participants. We know that small studies are problematic, and that the random play of chance can have a major influence on results [8] such that results with fewer than 200 actual events may mislead [9]. The best source of information will be from published systematic reviews and meta-analyses of randomised trials of interventions to improve quit rates with information after defined periods (six months, 12 months, longest), and in which placebo or no-treatment controls are a common comparator. 2. Methods Systematic reviews and meta-analyses were sought of interventions to increase quit rates. Two main searching strategies were employed: 1. Searching of Cochrane Library for Cochrane reviews, using key words tobacco, nicotine, alcohol, drinking, cocaine, heroin, opioids, and by examining lists of reviews from the appropriate review groups. 2. Searching PubMed, using similar key words in title or abstract, in humans, and in English. Related article links were also used. Only systematic reviews or meta-analyses were used. Where several examined the same subject, the most recent and largest was chosen. Analyses of predominantly short-term outcomes were excluded because these may not provide a reliable measure of longer-term abstinence. In general, articles with cessation rates after at least six months were chosen; some included a few studies with shorter duration (three months, for example), but the proportion of short duration studies was small. Those using randomised or quasi-randomised trials were preferred; in some conditions where there was limited evidence from randomised trials, some controlled trials that were not convincingly randomised had been included in the reviews, but the amount of information from this source was a small minority of the total available. These few reviews were included to provide information where otherwise none would have been available; this was mostly in interventions for cocaine abuse, where the number of subjects for any review was small. For example, non-randomised studies contributed fewer than 1% of patients in reviews of smoking cessation. The outcome for cessation trials was quit rates–typically the percentage of participants abstinent according to an objective test of abstinence at six months or longer. Results from each review are presented as absolute quit rates with treatment and placebo, together with a relative benefit or risk, and number needed to treat (NNT). We abstracted cessation rates for individual trials included in the reviews, and calculated overall cessation rates for intervention and placebo. For completeness, and to demonstrate that studies measuring placebo responses were sensitive in that interventions caused change in response, we also calculated relative risk with 95% confidence interval (CI) using the fixed effects models [10]; statistical significance was assumed when the 95% CI did not include one. Numbers needed to treat (NNT) compared with placebo with 95% confidence interval were calculated from pooled data [11] only with a statistically significant results.