Hierarchy of evidence and investigating causation
In public health, as in clinical medicine, we are concerned about using evidence to support our decisions and plans. Evidence can come from many sources, and we find some sources and evidence more trustworthy than others. We may prefer to accept evidence or information because it comes from someone we trust, such as a senior colleague, acknowledged expert or a family member rather than from a website, a newspaper or a commercial company interested in selling us their product. We may also recognise that some evidence is intrinsically more likely to be a reliable guide to action than others. A large, well-conducted randomised controlled trial (RCT) testing a new drug against current treatment is more likely to convince us of the efficacy or otherwise of the new drug than an anecdotal report from one patient that they improved after taking it. In other words, we want to base our decisions, if possible, on evidence that is as unbiased and least subject to confounding or chance findings (see Chapter 6) as possible.
The hierarchy of evidence reflects this relative weight or value given to the different research methods and study designs. This forms an integral part of evidence-based practice, particularly in making recommendations. In general, experimental studies are given greater weight than observational studies, which in turn have greater weight than case reports.
Aetiology refers to the study of causes of diseases. It is often difficult to understand aetiology because of the following reasons: