Outcomes are measured at specific times and any difference in response between the groups is assessed statistically. This method is also used to reduce bias. RCT See Randomised controlled trial . Receiver operated characteristic (ROC) curve A graphical method of assessing the accuracy of a diagnostic test. Sensitivity is plotted against 1 minus specificity. A perfect test will have a positive, vertical linear slope starting at the origin. A good test will be somewhere close to this ideal. Reference standard The test that is considered to be the best available method to establish the presence or absence of the outcome this may not be the one that is routinely used in practice. Reporting bias See Publication bias . Resource implication The likely impact in terms of finance, workforce or other NHS resources. Retrospective study A research study that focuses on the past and present. The study examines past exposure to suspected risk factors for the disease or condition. Unlike prospective studies, it does not cover events that occur after the study group is selected. Review question In guideline development, this term refers to the questions about treatment and care that are formulated to guide the development of evidence-based recommendations. Risk ratio (RR) The ratio of the risk of disease or death among those exposed to certain conditions compared with the risk for those who are not exposed to the same conditions (for example, the risk of people who smoke getting lung cancer compared with the risk for people who do not smoke). If both groups face the same level of risk, the risk ratio is 1. If the first group had a risk ratio of 2, subjects in that group would be twice as likely to have the event happen. 