Incidence is a dynamic concept that measures risk or the force of morbidity. It is a measure that deals with events and change in status (e.g from healthy to ill). Incidence time is the time at which new disease occurs in a population for example age at death. Incidence number is the number of new cases detected in the time interval for example number of cases of SARS detected in a period of 3 months. The term absolute rate is also used to refer to the number of new cases in a given time interval.

INCIDENCE RATE

The incidence rate (IR) is a basic measure of disease occurrence. Incidence rate (IR) = incident number/ total person-time. Person time is defined as the sum of the product of number of persons observed by the length of time that they are observed. The concept of person-time is difficult to understand and use and the following easier definition of incidence rate is used: Incidence rate = # newly reported cases of a disease in a year / mid-year population. The incidence rate has a lower bound of zero but has no fixed upper bound.

SURROGATE MEASURES OF INCIDENCE

In many cases we do not have data on the incidence of a new disease. The patients may not come to hospital. If they come the disease may be misdiagnosed or may not be reported. Some diseases occur but are clinically silent and the patient may die of another condition before the disease is known. We therefore cannot know for sure the total incidence of some diseases and we may have to rely on surrogate measures of incidence. The most commonly used measure is mortality rate. Mortality rate is defined as number of cases dying of a certain disease in a year / mid-year population.

DIFFERENT TYPES OF RATES: CRUDE, SPECIFIC, and STANDARD

A rate is the number of events in a given population over a defined time period and has 3 components: a numerator, a denominator, and time. A crude rate is based on direct computation of incidence for the whole population without taking into consideration sub-group variations. It is therefore misleading and unrepresentative. Inference and population comparisons based on crude rates are not valid. Rates can be specific for age, gender, race, and cause. Specific rates are more informative than crude rates but are cognitively difficult to internalize, digest, and understand so many rates or be able to reach some conclusions. An Adjusted or standardized rate is a representative summary that is a weighted average of specific rates free of the deficiencies of both the crude and specific rates. Standardization eliminates the ‘confusing’ or ‘confounding’ effects due to subgroups. Standardization of rates is achieved using mathematical formulas and procedures that we will not discuss in this preliminary course. In standardization, a ‘standard population’ is used to calculate a standard rate.  A standard rate unlike a crude rate can be used to compare different populations or countries with different underlying population structures.