A clinical trial involves the testing of a new medicine (or other therapy) to evaluate whether it is effective and safe. Clinical trials of drugs can be divided into those assessing the treatment of a disease (e.g. asthma) or those assessing treatments to prevent the occurrence of significant healthcare events in the future (e.g. stroke). Clinical trials provide the quantitative information about the benefits, adverse effects and possible uses of new drugs that allows prescribers and patients to make rational decisions in relation to drug therapy. The information will provide the relative risk reduction (RRR), absolute risk reduction (ARR), the numbers needed to treat (NNT) for one to be better off on treatment than comparator.