Incidence, in statistics, is the rate of occurrence of something or the number of times it occurs within a population at a particular time or time period.
The rate of incidence is usually expressed as a fraction, for example, 48 cases of measles out of a population of 750 schoolchildren over the course of a winter. The rate may also be expressed as a percentage, however. For example, the occurrence of data breaches within a certain time period might be categorized according to industry, in which case the rate of incidence could be expressed as a percentage of the whole. Within the United States in 2011, the rate of incidence of exposed records broke out as: government and armed services were accountable for 44 percent of all exposed records; non-financial businesses, 33 percent; medical and healthcare, 16 percent; educational institutions, 4 percent; and banking, credit and financial firms, 3 percent.
Here are a few more examples of how incidence is used in statistics:
The incidence of homicides in a particular city and year would likely be expressed as the number of such crimes. For example, there were 411 homicides in Detroit in 2012. (In that case, the number of the population may not be explicitly expressed.)
The incidence of identity theft in the United States might be expressed as a number or a percentage. For example, that rate rose three years in a row between 2010, 2011 and 2012. The incidence for those years was 4.35, 4.9 and 12.6 million people, respectively. Those figures might also be expressed as percentages. The incidence rate of 12.6 million for 2012 can be expressed as five percent of the population.
The number of hardware failures leading to system crashes per 1000 users within a given time period. Such figures might be used for a comparison -- for systems using Intel chips vs. AMD chips, for example, or for overclocked systems compared to underclocked ones or systems running at manufacturer-stipulated rates.
The churn rate, in a service provider context, measures the incidence of customers changing providers. If five out of twenty customers drop a given provider in a particular year, that provider has a churn rate of 25%.
The word incidence is sometimes confused with incidents because the two sound similar and have related meanings. However, an incident is a single occurrence -- an event. Multiple occurrences may be referred to as incidents, but the word doesn't imply anything about the rate of occurrence.