Introduction
This topic covers the two basic measures of disease frequency, incidence and prevalence, with their variants; the active and passive methods of collecting morbidity statistics; health indicators (HALE, QALY, DALY); and the International Classification of Diseases (ICD).
Incidence and prevalence
- Prevalence is a measure of the number of cases (old and new) at a point in time.
- Incidence is a measure of the number of new cases arising in a population over an interval of time.
Incidence
- Needs: new cases, population at risk, an interval of time.
- It is a measure of the frequency with which new cases of illness occur in a population over a period of time: the occurrence of an event or characteristic over a period of time.
- It is the rate at which healthy people develop a disease; it is a statement of probability. A high incidence rate means a high occurrence of the disease.
- It can be used to estimate the risk of an event during a stated period of time.
It is used to:
- describe the rate of disease occurrence over time (the time interval must be given)
- compare the risk of disease between 2 or more populations
- assess patient survival: incidence of death over time or at a specific time after follow-up
Variants of the incidence rate
- Attack rate: applied to a narrowly defined population observed for a limited time (during an epidemic); a variation of the incidence rate, usually expressed as %.
- Secondary attack rate: the frequency of new cases of a disease among the contacts of known cases, in %.
- Case fatality ratio: the measure (in %) of deaths due to a disease in a given period among all cases of the disease in the same period.