Measuring disease frequency. Comparing disease occurrence. Absolute and relative comparison (risk different, etiological fraction, population attributable risk, relative risk, odds ratio). (1/2)
Measurement in epidemiology
- The scope of measurements in epidemiology is very broad and unlimited. For example: measurement of morbidity, mortality, disability, natality, etc
- The epidemiologist usually expresses disease magnitude as a rate, ratio or proportion.
Rate
- A rate measures the occurrence of some particular event (for example development of disease or occurrence of death) in a population during a given time period
- Statement of risk of developing a condition
- Indicates the chance in some events that takes place in a population over period of time
- An example of a typical rate is the death rate.
- A rate comprises the following elements – numerator, denominator, time specification and multiplier. The time dimension is usually a calendar year. The rate is expressed per 1000 or some other round figure (10 000, 100 000) selected according to convenience or convention to avoid fractions.
Ratio
- It expresses a relation in size between two random quantities
- The nominator is not a component of the denominator
- The nominator and denominator may involve an interval of time or may be instantaneous in time. Broadly, ratio is the result of dividing one quantity by another.
Example:
Proportion
- A proportion is a ratio which indicates the relation in magnitude of a part of the whole
- The numerator is always included in the denominator
- A proportion is usually expressed as a percentage.
Example: