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The mean age of the alcohol sub-sample of the study sample was 40.4 (SD 12.6) and this concurs with the mean age for first time attenders at specialist agencies in the whole of Yorkshire of which the study site is a part, in the year immediately following the study recruitment period. The mean age for first time attenders with alcohol problems in the Northern region was 39. The mean age of the heroin sub-sample of the study sample was 24; the mean age of first time attenders seeking treatment for drugs other than alcohol in the Northern and Yorkshire regions was 25. Mean age specifically for the heroin help seekers was not reported (Pace 1998). Thus the mean age of the sub-samples in the study population was similar to that of the mean age of agency first time attenders in the wider region and the adjacent region.
Thom (1986) found no differences between men and women in the perceived duration of their alcohol problem prior to attending for specialist treatment; in the present study, no significant differences were found between men and women in their reported duration of either alcohol use or problem alcohol use in the alcohol sub-sample and none were found between men and women in the heroin sub-sample in their reported duration of use and of problem use (see Chapter 8). The reported duration of use and of problem use in the alcohol sub-sample were both far greater than those reported by the heroin users and this difference is consistent with what would be expected given the difference in mean age between the two substance groups.
10.2.3 Non-response bias
A further threat to the validity of the study findings is non-response bias. Attempts are made to minimise this source of bias by maximising the number of respondents contacted at follow-up and by maximising the amount of information available on those who did not participate in follow-up. In the present study, follow-up rates planned for the purpose of conducting statistical analyses of the findings were higher than those actually obtained.
Alcohol and drug misusers are notoriously difficult to trace at follow-up, the more so in the case of drug users than alcohol users (Cottler et al. 1996) and this was found to be the case in the present study. A number of reasons are proposed: this is often a highly transient group due to the financial and domestic harms caused by substance misuse; the illicit nature of some of the activities involved mean that people may not be keen to be traced or may be incarcerated in prison, and there is a relatively high mortality rate compared with the general population. Information regarding the study participants who were accounted for but did not participate in follow-up, described in Chapter 7, sections 7.3.3 and 7.3.4, lends support to this picture.
There was little opportunity to verify the address given at intake when individuals were seen 221