When searching for evidence to answer our clinical questions, the ability to rapidly recognise different types of studies is helpful for finding the one that best answers the question. “Levels of evidence” tables make suggestions for which design is best for which type of question. For instance, you would naturally consider a randomised controlled trial as the most appropriate study design for intervention decisions. But for potential harms of interventions, we may need case–control studies. And for aetiology, we often need to use cohort studies: you wouldn’t randomise someone to cigarette smoking to see if they did worse—that would also be unethical. But you would want investigators to follow up cigarette smokers and non-smokers for a long time, just as Richard Doll did.1

This short Notebook is a brief guide to the different study types and their advantages and disadvantages. In trying to understand why investigators chose a particular study type, several factors need to be taken into account. The first thing to recognise is that both clinical questions and study designs have similar components (as we’d …