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consultation during the recruitment year, 387 help seekers with primary opiate use and 289 help seekers with primary alcohol use were seen at the agency base, the site chosen for the study samp[le selection.
7.1.2 Sample selection and sources of potential bias
On the basis of the data presented in the foregoing sections, a potential source of bias in the sample selection was choice of the agency base for recruitment of all participants to the study. Compared to all help seekers at the agency, those seen outside the agency base are the very young, the very old, the house-bound for domestic reasons or for reasons of illness and immobility. These individuals will have been under-represented in the study sample which could be said to be representative of those who attend the agency base for their assessment and treatment; additionally, as a proportion of all those seen by the agency, the study sample has a higher proportion of heroin to alcohol users. Data on age and gender differences for the entire help seeking population seen at the agency base as opposed to the community were not available.
7.1.3 Sample Size
Recruitment rates for new patients attending the Leeds Addiction Unit were examined in order to establish a realistic sample size that could be obtained at the initial data collection point and at two follow-up points within existing time and resource constraints, as well as satisfying minimum requirements of the data analysis. It was estimated that the largest number of items in the item pool of any of the planned multi-variate analyses would be 19 and for this an ideal sample size of 190 should be obtained for those tests which required a 10:1 subject-item ratio, though a subject-item ratio of 5:1 may be satisfactory (Nunnally 1978). It was thought to be unrealistic within given resources to follow up 190 individuals at three months and twelve months; to do so would have required an initial recruitment of 317 individuals and time constraints were thought to preclude this figure. Further discussion of sample selection and sample size is in Chapter 10.
I decided, on the basis of available resources, to attempt to recruit 100 participants at both the follow-up points and that this figure would constitute 60% of the original sample. A minimum of 166 participants was therefore the target for recruitment at the first data collection point. Sixty per-cent was thought to be a realistic proportion for which to aim in view of the difficulties reported in following up this sort of population in the absence of considerable research funds (Cottler et al. 1996). Routine follow-up at the Leeds Addiction Unit up to the time of the study was conducted by postal questionnaire and yielded a follow-up rate of 25%.
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