BMC Public Health 2005, 5:75 http://www.biomedcentral.com/1471-2458/5/75 Methods The MATCH Data set was made available to qualified researchers after the study had been completed. As an NIAAA committee member designing another large multi- center funded alcoholism study, the senior author of this study (RBC) obtained an official copy of the Project MATCH Public Data Set (V1.0) from the Coordinating Center in Farmington, Connecticut on August 24, 1998. The trial was conducted with approval from the appropri- ate ethics committees, with informed written consents, and the procedure was in compliance the Helsinki Decla- ration. This data set comprised some 256 variables on all 1726 participants in Project MATCH. Drinking data were summarized for each participant at pre-treatment, at weekly intervals for the 12 weeks of the trial, and at monthly intervals during the follow-up. Drinking data consisted of two measures, percent days abstinent (PDA) and drinks per drinking day (DDD). Participants were identified by which of the three treatments they received, Cognitive Behavioral (CBT), Motivation Enhancement (MET) or Twelve-Step (TSF). The number of treatment ses- sions each participant attended was also included. It is this variable that is the focus of this study. The first step of the analysis was to compare our data to official published results. Pearson correlations were com- puted between number of treatment sessions attended with percent days abstinent and with drinks per drinking days at follow-up. We then computed correlations between number of sessions and the drinking variables during treatment. Both our analyses and previously pub- lished MATCH analyses substituted transformed scores for the drinking measures in order to normalize the data and thereby meet the assumptions of the statistical tests. The transformed scores were supplied in the data set. In this study we used the transformed scores for all statistical tests and the original drinking measures for display purposes. In the next step the participants were categorized into groups based on the number of treatment sessions they attended, ranging from 0 to 12. Those alcoholics who signed up for the study but who never attended any ses- sions were coded zero. Those who were coded 12 attended every session and received either CBT or TSF but not MET. MET consisted of only 4 sessions. The outcome variables analyzed in this study are restricted to the two primary MATCH outcome variables. Three groups were formed consisting of participants who attended either 0, 1 or all 12 treatment sessions. Chi square tests were used to test relationships between cate- gorical variables. Repeated measures Manovas were used to test the differences between groups over all time points. Drinking outcome at follow-up was a mean of the entire follow-up period (from month 4 to month 15). The trans- formed scores were used to compute effect size. Paired t- tests were used to test for change within groups and inde- pendent groups t-tests were used to test differences between groups. Drinking data were incomplete for a number of the participants in the 0 and 1 session treat- ment groups. The highest level of missing data occurred at week one, where data were available for 57% of the 0 ses- sion group and 73% of the 1 session group. By follow-up, data were available for about 80% of participants in both these dropout groups. Results Table 2 compares our results to the Project MATCH's pub- lished results [20] on the correlations between number of treatment sessions attended (0 to 12, or 0 to 4 for MET) and the two primary outcome measures, drinks per drink- ing day and percent days abstinent. The table shows that our results are essentially identical to the official results. This indicates that the data used in the present study are correct. Additionally the results show rather low correla- tions between number of treatment sessions attended and outcome, particularly long term outcome. The percent of variance explained can be obtained by squaring the corre- lation coefficient. The amount of outcome that can be attributed to attending treatment ranges from 0 to 9%, with a median of approximately 3%. Table 3 displays the same type of correlations as in Table 2, but this time the drinking measures were for the weeks during treatment. The table shows that there is a relation- ship between drinking level at these early time points and number of treatment sessions. Note that drinking level at week one predicts the total number of weeks the partici- pant will remain in treatment. Tables 4 and 5 display the mean follow-up data for the two primary drinking outcomes (percent days abstinent and drinks per drinking day) of Project MATCH. Outcome at follow-up is a mean of the data from 4 to 15 months. Overall the data show that the three treatments were fairly equal and that patients who attended more sessions had somewhat better outcomes. However, there was one anomalous group. Those who dropped out after one ses- sion (the 1 treatment group) had worse outcomes than those who dropped out before attending even one ses- sion. They had worse scores than this zero treatment group on 46 of 50 measures (percent days abstinent and drinks per drinking day at pre-treatment, 12 weeks of scheduled therapy, and 12 follow-up time points). All the data are shown in the attached file [see Additional file 1]. They also had worse scores than the other treatment groups (those who attended 2 to 12 sessions) on 526 of 550 measures. The one treatment group was significantly worse that the zero treatment group over all time points Page 3 of 11 (page number not for citation purposes)