When appraising research, "three things to bear in mind are quality, validity, and size:
Trials that are randomised and double blind, to avoid selection and observer bias, and where we know what happened to most of the subjects in the trial.
Trials that mimic clinical practice, or could be used in clinical practice, and with outcomes that make sense. For instance, in chronic disorders we want long-term, not short-term trials. We are [also] ... interested in outcomes that are large, useful, and statistically very significant (p < 0.01, a 1 in 100 chance of being wrong).
Trials (or collections of trials) that have large numbers of patients, to avoid being wrong because of the random play of chance. For instance, to be sure that a number needed to treat (NNT) of 2.5 is really between 2 and 3, we need results from about 500 patients. If that NNT is above 5, we need data from thousands of patients.
These are the criteria on which we should judge evidence. For it to be strong evidence, it has to fulfil the requirements of all three criteria."
Source: Critical Appraisal. Bandolier.
Critical appraisal is the process of carefully and systematically examining research to judge its trustworthiness, its value, and its relevance in a particular context.
Information, calculators, checklists, and tools to help you critically appraise research articles are provided on this page.
Check out a copy of Trisha Greenhalgh's How to Read a Paper to read about how to find a medical research paper, assess it for its scientific validity, and where relevant, put the findings into practice.