Clinical Trials

Clinical Trials: A Practical Guide ■❚❙❘Meta-analysisThe systematic review and evaluation of the evidence from two or moreindependent studies asking the same clinical question to yield an overall answerto the question.Normal distributionA bell-shaped symmetric distribution for a continuous variable with the highestfrequency at a mean value and the lowest frequency further away from this meanvalue. A normal distribution can be completely described by two parameters:mean (μ) and variance (σ 2 ). In the special case of μ = 0 and σ 2 = 1, it is called thestandard normal distribution.Number needed to treat (NNT)This term is often used to describe how many patients would need to be given atreatment to prevent one event. It is determined from the absolute differencebetween one treatment and another. For example, in a randomized study, a groupreceiving treatment A had a death rate of 12.5% and a group on treatment Bhad a death rate of 15.0% in groups matched for size and length of follow-up.Comparing the two treatments there was an absolute risk reduction of15% – 12.5% = 2.5% for treatment A. From this we can derive that the NNT(= 1/0.025) is 40. This means 40 patients need to be given treatment A rather thantreatment B to prevent one additional death.Odds ratio (OR) and risk ratio (RR)These terms compare the probability of having an event between two groupsexposed to a risk factor or treatment. The risk ratio (RR) is the ratio of theprobability of occurrence of an event between two groups. The odds ratio (OR) isthe ratio of patients with and without an event in each group. For example, if thenumber of deaths in the treatment and control arms (both of sample size 100) ofa randomized study are 50 and 25 respectively, the RR = (50/100) / (25/100) = 2.The treatment group has a 2-fold relative risk of dying compared with the controlgroup. The OR = (50/50) / (25/75) = 3, indicating that the odds of death in thetreatment arm is 3-fold that of the control arm.457