PRINCIPLES OF TOXICOLOGY

time. The incidence rate is the classic measure of disease rates in an epidemiologic study. For example,
if four out of the five lung cancer cases developed in the asbestos-exposed group of workers discussed
above over the course of one year, then the incidence rate for lung cancer in that worker population
would be 4/1000 per year or 0.4. The incidence rate is different from the prevalence since only persons
who develop a disease during a set period of time are included. Therefore, incidence rates and
prevalence cannot be directly compared. Incidence rates are the measure of disease frequency in cohort
studies.
21.8 MEASUREMENT OF ASSOCIATION OR RISK
21.8 MEASUREMENT OF ASSOCIATION OR RISK 517
In epidemiologic studies, populations with different frequencies of disease or exposure are often
compared to suggest or establish an association between particular diseases and exposure. This is also
described as the risk of disease from a particular exposure. For example, if the incidence rate of lung
cancer in a worker population without asbestos exposure is 1/1000 workers per year, this can be
compared to the rate of lung cancer in an asbestos exposed population of 4/1000 per year. The
comparison shows that the rate of lung cancer is higher in the asbestos-exposed population than the
unexposed population. This suggests an association between asbestos exposure and lung cancer, or
that there is a risk of lung cancer with asbestos exposure.
As mentioned above, prevalence is the measurement of disease or exposure frequency in
cross-sectional studies. The risk of disease can be examined using the prevalences of disease in the
exposed and unexposed populations by formulating the rate ratio and the rate difference (Table 21.3).
The rate ratio is the ratio of the prevalence of a particular disease in the exposed population to the
prevalence of the unexposed population. The rate difference is the difference between the prevalence
of a particular disease between the exposed population and the unexposed population. A rate ratio
greater than or less than one indicates increased or decreased risk of disease; a rate difference greater
than 0 indicates increased risk of disease. These measures of risk can be tested for statistical significance
using the chi-square (χ 2 ) test and a confidence interval.
The incidence rate is the measurement of disease or exposure frequency in cohort studies. The risk
of disease in the exposed and unexposed population is examined with incidence rates of disease in the
exposed and unexposed populations using the rate ratio and the rate difference (Table 21.4). The rate
ratio is the ratio of the incidence of a disease in the exposed population to the incidence in the unexposed
population. The rate difference is the difference between the incidence of a particular disease between
the exposed population and the unexposed population. These measures of risk can also be tested for
statistical significance using the χ 2 test and a confidence interval.
Rate ratios are often standardized to create a standardized rate ratio (SRR) so that rates from
different populations are compared after eliminating confounding (see text below). For example, if
one of the populations is older, then age as a confounder can lead to a false conclusion when comparing
cancer disease rates in the two populations; the older population will inevitably have a higher cancer
TABLE 21.3 Prevalence Data: Rate Ratio and Rate Difference
Disease
+ –
+ A B
Exposure
– C D
Prevalence of disease in exposed population = A/(A + B)
Prevalence of disease in the unexposed population = C/(C + D)
Rate ratio = A/(A + B) divided by C/(C + D)
Rate difference = A(A + B) – C/(C + D)