The main quantitative research designs are reviewed, to indicate their strengths and limitations. Even the best designed and conducted studies run the risk of being influenced by the ever-present threats of chance, bias, and confounding, terms that are explained and illustrated with examples, and of sampling and measurement error. The informed reader should therefore look beyond the results of a study, using critical appraisal skills to ask whether the study’s design, sample, and measures are robust enough to allow the findings to be taken as likely to be accurate.