Читать книгу Statistics in Nutrition and Dietetics - Michael Nelson - Страница 46

In epidemiology, experimental studies take two forms: clinical trials and community trials. The aim is to compare the impact of the active treatment on the relevant outcomes with the impact of a placebo.

Clinical trials in epidemiology usually take the form of randomized controlled trials. The aim is to see if an intervention to alter exposure results, over time, in a change in outcomes. In epidemiology, clinical trials can involve thousands of subjects followed over many years, whereas studies relating to drug treatments or food interventions relating to changes in blood or urine biochemistry may involve only tens of subjects. The purpose in having such large studies in epidemiology is to be able to generalize to the population with confidence, and to take into account the many confounders (see below) that may operate in the real world.

Researchers usually strive to achieve ‘double blind’ status in their study designs, but in nutritional interventions this may not always be possible. If the intervention involves changing diet, for example, or providing nutritional advice to increase fruit and vegetable consumption, the subject may be aware of the changes and is therefore no longer ‘blind’. Similarly, the researcher involved in administering the changes may be aware of which subjects are in the treatment group and which are in the placebo group. If this is the case, it is important to ensure that the person undertaking the statistical analysis is blinded to the identity of the groups being compared. This can be done through the coding of results for computer analysis so that the comparison is simply between group A and group B. Only at the end of the analysis is the identity of the group revealed. Even here, it may not always be possible to blind the analyst. In that case, the procedures for carrying out the statistical analyses should be described in advance so as to avoid a ‘fishing expedition’, hunting for statistically significant findings.

Community trials are intervention studies carried out in whole communities to see if some change in exposure is associated with a subsequent change in disease or mortality rates. Again, the study will involve a treatment community and a placebo community. The communities are matched (e.g. age and sex structure of the population, percentage of the population not in work). There may be more than one community in each group.

Community trials are pragmatic in nature. The aim is to see if community‐based interventions have sufficient penetration and impact to bring about changes in nutrition‐related outcomes (e.g. the percentage of adults over the age of 45 who are overweight or obese). The identity of the individuals in the community who make the desired changes and their individual outcomes may not be known. A questionnaire can be used to find out the proportion of the population who were aware of the intervention (for example, advice on healthy eating provided in a GP surgery by the community dietitian), and height and weight data could be taken from the routine ‘healthy man’ or ‘healthy woman’ screening programmes being run in the same groups of GP surgeries.

Community trials are much cheaper to run than clinical trials. They do not, however, provide the same wealth of detail about individual behaviours and outcomes and, as a consequence, provide less insight into causal mechanisms.