"Critical appraisal is the process of carefully and systematically examining research to judge its trustworthiness, and its value and relevance in a particular context."
Different types of clinical questions are answered by different types of study design. Here is an overview of the most common study designs:
Randomised Controlled Trial (RCT): Used to answer questions about effects. Participants are randomised into two (or more) different groups and each group receives a different intervention. At the end of the trial, the effects of the different interventions are measured. Blinding (patients and investigators should not know which group the patient belongs to) is used to minimise bias.
Cohort study: Participants or subjects (not patients) with specific characteristics are identified as a 'cohort' (cohort=group) and followed over a long time (years or decades). Differences between them, such as exposure of possible risk factor(s), are measured. Used to answer questions about aetiology or prognosis. Prognostic cohort studies start with a group of patients with a specific condition and follow them up over time to see how the condition develops.
Case-control study: Looks at patients (cases) who already have a specific condition and match them with a control group who are very similar except they don't have the condition. Medical records and interviews are used to identify differences in exposure to risk factors in the two groups. Used to answer questions about aetiology, especially for rare conditions where a cohort study would not be feasible.
Cross-sectional study/survey: A representative sample of a population is identified and examined or interviewed to establish whether or not a specific outcome is present. Used to answer questions about prevalence and diagnosis. For diagnostic studies, the sensitivity and specificity of a new diagnostic test is measured against a 'gold standard' or reference test.
Qualitative study: Interviews, focus groups, participant observation. Used to answer questions about why people do what they do and how they feel.
Greenhalgh, T. (2010). How to read a paper: the basics of evidence-based medicine. Chichester, West Sussex, UK, Hoboken, NJ: Wiley-Blackwell.
Hoffmann, T., Bennett, S., & Del Mar, C. (2009). Evidence based practice across the health professions. Chatswood, N.S.W.: Elsevier Australia.
The Critical Appraisal Skills Programme (CASP) have made available check lists for the critical appraisal of different study types.
The AGREE Collaboration has a check list for the critical appraisal of clinical guidelines.