This may be best measured by the health benefits that could have been achieved had the money been spent on the next best alternative healthcare intervention. Outcome The impact that a test, treatment, policy, programme or other intervention has on a person, group or population. Outcomes from interventions to improve the public s health could include changes in knowledge and behaviour related to health, societal changes (for example, a reduction in crime rates) and a change in people s health and wellbeing or health status. In clinical terms, outcomes could include the number of patients who fully recover from an illness or the number of hospital admissions, and an improvement or deterioration in someone s health, functional ability, symptoms or situation. Researchers should decide what outcomes to measure before a study begins. P value The p value is a statistical measure that indicates whether or not an effect is statistically significant. For example, if a study comparing 2 treatments found that one seems more effective than the other, the p value is the probability of obtaining these results or more more extreme by chance. By convention, if the p value is below 0.05 (that is, there is less than a 5% probability that the results occurred by chance) it is considered that there probably is a real difference between treatments. If the p value is 0.001 or less (less than a 1% probability that the results occurred by chance), the result is seen as highly significant. If the p value shows that there is likely to be a difference between treatments, the confidence interval describes how big the difference in effect might be. Perioperative The period from admission through surgery until discharge, encompassing the preoperative and postoperative periods. 