A way of expressing the chance of an event taking place, expressed as the number of events divided by the total number of observations or people. It can be stated as ‘the chance of falling were one in four’ (1/4 = 25%). This measure is good no matter the The number of new occurrences of something in a population over a particular period of time, e.g. the number of cases of a disease in a country over one year. of events i.e. common or infrequent.