What types of questions can be asked?

Clinical problems and health policies may involve many different questions which need to be informed by the best available evidence. It is useful to have a classification of the different types of health care questions that we may ask:

• Phenomena: ‘What phenomena have been observed in a particular clinical problem, e.g. what problems do patients complain of after a particular procedure?’

• Frequency or rate of a condition or disease: ‘How common is a particular condition or disease in a specified group?’

• Diagnostic accuracy: ‘How accurate is a sign, symptom or diagnostic test in predicting the true diagnostic category of a patient?’

• Aetiology and risk factors: ‘Are there known factors that increase the risk of the disease?’

• Prediction and prognosis: ‘Can the risk for a patient be predicted?’

• Interventions: ‘What are the effects of an intervention?’

Answering each type of question requires different study designs, and consequently different methods of systematic review. A thorough understanding of the appropriate study types for each question is therefore vital and will greatly assist the processes of finding, appraising and synthesizing studies from the literature. A summary of the appropriate study types for each question and of the issues that are important in the appraisal of the studies is also given in Table 1.1. General information on how to find and review studies is given in the remainder of Part 1 with further details for each question type in Part 2.

Interventions

An intervention will generally be a therapeutic procedure such as treatment with a pharmaceutical agent, surgery, a dietary supplement, a dietary change or psychotherapy.