Learning Objectives
To define and provide examples of a population
To distinguish between a fixed and dynamic population
To describe the key aspects of measuring disease occurrence
To define and distinguish among prevalence, cumulative incidence, and
incidence rate
To describe the mathematical relationship among the measures of disease
frequency
To calculate and interpret commonly used measures of disease frequency in
public health
To know how to calculate person-time
To organize disease frequency data into a two-by-two table
To describe and calculate absolute and relative measures of comparison
including rate/risk difference, population rate/risk difference, attributable
proportion between the exposed and total population, and the rate/risk ratio
(relative risk)
To be able to interpret each absolute and relative measure of comparison
To describe the purposes of rate standardization and calculate the directly-
adjusted standardized rate, and SMRs using indirect standardization
disease frequency in populations
through the calculation of rates, we have to actually measure the frequency of disease in a
population before we can look at its distribution geographically or by other factors
helps identify patterns of disease and also disease trends in a population as they occur
short-term and long-term trends are all based upon the calculation of rates
A population in general is a group of persons with a common characteristic
range from living in the same city, to being born in the same year, to being in the same
classroom
there are some rates that are more appropriate to a fixed population and some rates more
appropriate to a dynamic population
fixed population
once you're in that population, you're in it forever until you essentially exit it through death
Examples members of the Framingham cohort study of Massachusetts, persons who were living
around the Chernobyl reactor in Ukraine in 1986 and were exposed to radiation, and also
individuals living in Hiroshima and Nagasaki, Japan, and were exposed to the atomic bombings in
both cities.
Their exposure fixed them in those two populations forever
dynamic population
more common
people enter and leave - birth, deaths, migration, immigration. It's in constant flux/changing
o
Ex. the population of most countries or most cities or most states are defined as
dynamic populations
the mechanics of actually how we calculate rates in a population
need three pieces of information

1.
a numerator, the number of cases of a particular disease that occurs in a population
2.
a number of the population itself, or a denominator
3.
need to account for that time to the calculation of person time - in the case of the only true rate,
an incidence rate, a function of time, or a person time, because the rate of disease changes as a
person moves through time
Three Types of Frequency Measures
Used to relate number cases of disease, size of population, and time.