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not self-report is accurate (likely to be an unanswerable question), it may be of greater value to investigate the validity and reliability of self-report in help seekers and other groups of substance users. In the following sections the factors affecting self-report are described, the evidence for its reliability and validity is reviewed and some methods for enhancing these are presented.
3.2 The reliability of responses
Recall and social desirability were found by Embree and Whitehead (1993) to be salient factors in the reliability and validity of self-reported drinking behaviour; they proposed that questions designed to aid recall and mitigate the effects of social desirability would enhance the reliability and validity of responses.
3.2.1 Recall
Self-report relies heavily on recall. Hammersley (1994) has examined the psychological and social factors influencing recall and made specific proposals for the handling of self-report data whether this is for current events, attitudes or beliefs or based upon recall of past events. He describes the general phenomena affecting self-report: the storage and retrieval of information are active processes subject to the influence of current social, cognitive and affective demands; the result is that there is no such thing as storage of information in a pure form. Information is encoded and stored differently according to what is going on at the time. The same is the case for recall. The memory of events will be influenced by the circumstances in which they are recalled, the emotional value or meaning of the events and the influence of events that have subsequently occurred. In other words there is a process of interpretation going on which shapes the way events, feelings and thoughts are both stored and retrieved.
Several findings reviewed by Hammersley (1994) have particular relevance for drug and alcohol research. Memory is in general poor for numerical data, timing, dates and age. The routine is less well remembered than vivid or unusual events. A memory may be changed by the thoughts about the event that the person has each time they recall that event. The requirements of the recall situation can shape the way the memory is presented and therefore the way that it is perceived by the person presenting it. In addition to this, a number of drugs are known to impair attention and concentration, performance and recall. Alcohol (Ron 1983, Tarter and Schneider 1976), benzodiazepines (Curran 1986), and cannabis (Solowij 1998) have been shown to impair memory in both the short and the long term. Both benzodiazepine (Rubin and Morrison 1992) and cannabis
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