"Critical appraisal skills enable you to assess the trustworthiness, relevance and results of published papers so that you can decide if they are believable and useful."
http://www.casp-uk.net/criticalappraisal
The critical appraisal process provides clinicians with the tools to interpret the quality of studies and determine the applicability of the synthesis of multiple studies' results to their patients.
The validity of a study refers to whether the results of the study were obtained via sound scientific methods. Bias (defined as the systematic deviation from the truth) and/or confounding variables may compromise the validity of the finding. The reliability of the study's results are determined by the size of the intervention's effect (the effect size) and how precisely that effect was estimated. This part of critical appraisal examines the numerical data reported in the results section of a study. When critically appraising the the usefulness of a study for clinical decision making, a basic aspect of applicability is to evaluate the study's patients in comparison with the patients to whom the evidence would be applied.
Many resources are available to help with the critical appraisal process.
These articles are from the first Users Guide series published in JAMA in 1994:
These calculators were developed by the Knowledge Translation Program in Toronto. They include calculators for statistics and Odds Ratio to NNT Converter.
This calculator can determine diagnostic test characteristics (sensitivity, specificity, likelihood ratios) and/or determine the post-test probability of disease given given the pre-test probability and test characteristics.
Population Prevalence, Sensitivity, Specificity, Predictive Values, & Likelihood Ratios
Predictive Values and Likelihood Ratios