Measures of the Frequency of Disease
Epidemiology is all about measuring disease frequency and factors that affect diseases and health status. There are several different measures of disease frequency. Careful selection of the population to include in the numerator and denominator of rates has a significant influence on the rate.
A rate is the number of events per unit of population in a set time span.
Population at risk:
This is the population who is potentially susceptible to the disease.
Prevalence and Incidence:
Prevalence – the number of existing cases at a specified point in time
Incidence – the number of new cases during a specified time period
The ratio of the population affected by a disease to the population at risk at a specified point in time.
The numerator is the number of people with the disease or condition at a specified time. The denominator is the number of people in the population at risk at the specified time.
Prevalence rate is increased by: - long disease duration; prolongation of life of cases without cure; increase in incidence; in-migration of cases; out migration of healthy people; in-migration of susceptible cases
Prevalence rate is decreased by: - short disease duration; high case fatality rate from disease; decrease in incidence; out-migration of cases; improved cure rate of cases.
The numerator is the number of persons who get the disease in a specified period and the denominator is the sum of the length of time during which each person in the population is at risk.
Units include a time dimension.
Cumulative incidence rate or risk:
In contrast to incidence rate, this measures the denominator at only one point in time, usually at the beginning of the study.
Numerator is the number of persons who get the disease during a specified period. The denominator is the number of persons free of the disease in the population at risk at the beginning of the period.
Usually presented as a rate of number of cases per 1000 population over a time period.
Risk Ratio (relative comparison; relative risk):
Commonly used. This is the ratio of the risk of occurrence of disease between two groups – usually the exposed and unexposed groups. The rate of one group is divided by the rate of the other group. As it is a ratio, it has no units.
This is a better measure of the strength of an association than risk difference as it is expressed relative to a baseline level of occurrence.
In case control studies
Attributable risk (risk difference; excess risk):
This is the rate of a disease or outcome in exposed individuals that can be attributed to an exposure – determined by dividing the risk difference by the rate of the occurrence among the exposed group.
If the exposure is believed to be the cause of a disease, the attributable risk is the proportion of the disease in that population that would be eliminated if the incidence of the exposure in the exposed group was reduced to the level in the unexposed group.
Population attributable risk:
This is a measure of the excess rate of disease in the total study population that is attributable to the exposure. Calculated by multiplying the risk difference by the proportion of the population exposed.
Numerator is the number of new cases of nonfatal disease. Denominator is the population at risk
Numerator is the number of deaths from a disease. Denominator is the total population.
Standardisation of rates:
Rates calculated with the numerator being the total number of events and the total population as the denominator are called crude rates – they are of limited value if comparing two populations if they have, for example, different age structures. The data can be adjusted to take this in to account by age standardisation.
Direct standardisation – includes a weighted average of the age specific rates in each year, with weights equal to the proportion of persons in each age group in a convenient reference population
Indirect standardisation – uses fixed or well defined age-specific rates and asks how many deaths would be expected in the population under study if these rates where to apply