Topic 1: Accounting of disease events
The use of rates and ratios
A rate is used in epidemiology to measure the frequency of events (cases) as they occur over time. Rates express the probability of occurrence of some event.
A rate is of the form:
b = (r/n) *c
where:
r = the number of cases that occur during a specified interval of time;
n = the total number of animals in the population at risk of becoming cases during the same interval. Note that r (the numerator) is a portion of n (the denominator).
c = represents a constant population size (such as per 1000, 100,000 animals etc.)
b = the rate expressed per unit time (eg.monthly, yearly)
A proportion is of the same form as a rate; (c being equal to 100 or % since proportions are usually expressed as percent  %).
An index is an estimate of a rate and is used when it is impossible to count directly the population at risk (n). The denominator is then obtained by counting some parameter which approximates the population at risk.
A ratio (k) expresses the relationship between a numerator and a denominator that does not include the numerator. For example:
k = a/b
where:
k is a ratio
a is the numerator
b is the denominator (does not necessarily include the numerator)
The two categories of rates most commonly used are:
1. those measuring disease events (morbidity rates), and
2. those measuring deaths (mortality rates).
