Relevant circumstances

Consider the relevance of the circumstances in which the health actions were compared.

Some studies aim to find out if a treatment works at all. Often, in these types of studies, the treatment is tested under ideal circumstances. The results of these studies may overestimate the benefits that the treatment would have under more real-life circumstances and can be misleading when making a decision about using a treatment.


Some studies are designed to find out if a health action can work under ideal circumstances, for example, with people who are most likely to benefit and most likely to follow treatment instructions, and with highly trained practitioners who deliver the treatment exactly as intended. These comparisons, which are sometimes called explanatory studies, may not reflect what happens under usual circumstances.

Studies that are designed to find out if health actions work under usual conditions are called pragmatic studies.  Far more explanatory studies are published than pragmatic trials, although many studies are explanatory in some ways and pragmatic in other ways. Explanatory studies are often favoured by pharmaceutical and medical device manufacturers that sponsor these studies because they are less costly and easier to carry out. In addition, there is a lack of public funding for large pragmatic trials. This means that the relevance to real-life decisions of many published studies is reduced.


The North American Symptomatic Carotid Endarterectomy Trial is an example of an explanatory study. It demonstrated that, in a group of highly selected patients with severe narrowing of the large arteries in the neck (carotid stenosis), the chance of having a stroke was reduced following a surgical procedure called endarterectomy. Patients, who before the procedure, had a high chance of having a stroke were most likely to have a favourable response to the surgery. Participating patients were closely followed and were operated on by skilled surgeons in specialist hospitals who followed a strict protocol. The trial showed that endarterectomy reduced the chance of stroke under those ideal circumstances, but it did not provide a reliable estimate of the beneficial and harmful effects of endarterectomy under more typical circumstances.

Remember: The findings of comparisons of health actions that were conducted under ideal circumstances may not be relevant to more typical circumstances.

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