Design principles

The impact of study findings is largely attributable to the hierarchy of study design. While it is widely accepted that experimental studies are more rigorous than observational studies, experimental studies can also be prone to bias—an important consideration to take into account when determining the impact and validity of your study findings. There are known biases associated with each study type, and it is therefore important to consider how bias could be introduced into your study, and how to control for bias when possible. While study methods can be ranked according to evidence hierarchy, it is worth acknowledging that each study type has its own merits, with some of the less experimentally rigorous study designs, such a case series or cohort studies, still serving as efficient study approaches to establishing evidence in support of a hypothesis to guide further investment in more costly experimental studies. Equally, the generalization of study findings does not always necessarily equate with the hierarchy of study design. Indeed, randomized controlled trials are often limited in the generalization of their findings. The hierarchy of study design is important both in planning your study and in interpreting the applicability of your results. In this chapter, we will rank study methods according to evidence hierarchy and review how study design impacts the level of evidence presented and generalization of findings.