The ideal study would use data on actual prescribing before and after documented exposure to promotion whilst keeping other influences on prescribing constant. This situation is very difficult to create in real life. Instead researchers have often relied on self-assessments of exposure to or reliance on promotion, and self-reported prescribing. They have also found it hard to measure changes over time, and have often used data on different practitioners to guess at such changes. This has serious limitations.
This section examines various approaches to the question of how promotion affects individual prescribing. The first approach uses self-reported reasons for changes in prescribing, and investigates whether promotion is one of these self-reported reasons94-98. The prescribing changes might be measured (i.e. externally verified) or they might be self-reported. Ideally there are specific changes in prescribing particular drugs. It is inherent in this approach that the exposure to, and relative influence of promotion is self-assessed. Consequently, with this approach it is difficult to do more than give practitioners opportunities to present researchers with their self-image as people who are, or are not, influenced by promotion.
Stronger evidence for some kind of association between promotion and individual prescribing decisions comes from studies that look for associations between variations in prescribing decisions and variations in reliance on promotion. In these studies doctors are asked general questions like how reliable or useful promotional information is, and/or whether it is important in their prescribing decisions. Prescribing by those who give more positive assessments of promotion is then compared with prescribing by those who are more sceptical. Prescribing data are either self-assessed, elicited in response to hypothetical situations, or real prescribing data are used. There is strong consensus from these studies that doctors who rely more on promotion are heavier or less rational prescribers, or adopt new medicines earlier than those who rely less on promotion99-108. However, this kind of research cannot show a causal connection between promotion and prescribing. The results may be confounded by other factors, such as the practice setting or method of payment. Furthermore, these studies cannot establish a temporal relationship between use of promotion and inappropriate prescribing: doctors who are already poorer prescribers may be the ones who rely on promotion, or a reliance on promotion may lead to poorer prescribing. Therefore, this body of research does not prove that if these doctors relied less on promotion their prescribing would improve.
A third group of studies looks at different levels of exposure to promotion (between doctors or over time), and prescribing. These studies look at specific drugs, and the promotion related to them. These are the best kind of evidence that promotion actually causes changes in individual prescribing behaviour. The studies described by Peay and Peay109, Orlowski and Wateska110 and Gönül et al.111 are rather convincing, and worth replicating in other situations and with other drugs. This would considerably strengthen the argument that exposure to promotion causes prescribing changes. Other studies of this kind are also somewhat suggestive, but have methodological shortcomings such as the possibility of a recall bias, uncertainty about generalizability, and reliance on selfreporting of prescription, or do not give enough methodological details, such as the method of selecting doctors to be surveyed, to allow evaluation112-116.
This section ends with a discussion of the effect of samples on prescribing. This is discussed separately because it presents different methodological challenges, so different approaches have been used.