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19 and 25 all of which refer to slowing down and cutting down drug use, would be readily adaptable to heroin users, the interviews with 10 heroin users at different stages of their using career were recorded. Four respondents said that cutting down and slowing down use referred to within session reduction, that is, using a smaller quantity of the drug and five respondents said that cutting down and slowing down referred to between session reduction, or using on fewer occasions per day or per week. One respondent said it could mean either or both. These responses were consistent with those to be expected from the alcohol sample and did not appear to perform differently from other items in the relevant parts of the scale. None of the respondents interviewed reported any difficulty in understanding these items with reference to heroin use.
5.7.7 Content validity
Content validity refers to the way a scale measures the entire domain of content;
“Rather than test the validity of measures after they are constructed, one should ensure validity by the plan and procedures of construction.....content validity rests mainly on appeals to reason regarding the adequacy with which important content has been sampled and on the adequacy with which the content has been cast in the form of test items” (Nunnally 1978 p. 92, 93)
In this chapter the construct impaired control has been discussed, its various meanings examined and current understanding described. In the original Impaired Control Scale, the understanding of impaired control as referring both to perceived inability to abstain and perceived inability to stop once started was applied. This domain of meaning and the balance of content between these components were retained in the adapted scale. The adapted scale is thought to have good face validity in that it appears to measure what is commonly understood to constitute impaired control and the items were reported to make sense by respondents in the pilot phase of adaptation.
5.7.8 Concurrent validity
Impaired control might be expected to predict the extent of use of the substance in that those who showed greater impaired control would be likely to use more of the substance than those who
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