372 EPIDEMIOLOGY
of disease and does not allow these rates to be compared to
other studies (Lange, 1991).
The overall mortality rates increase sharply with age after
puberty (Figure 1): the increase is in fact close to exponential
in its shape, as is clear from its linear form when plotted on a
logarithmic vertical scale (Figure 2). Consequently, if one of
the two populations to be compared has a greater proportion
of the elderly than the other, its crude rate will exceed the
other, even if their age-specifi c rates are identical throughout
the age range. The crude rate is the ratio of the total deaths
to the total population (this may be for both sexes together
or separately by sex), and more deaths will result from the
larger population of the elderly groups. However, it is pos-
sible to obtain a legitimate comparison using a single fi gure
for each population by the simple method of applying the
separate age-specifi c rates observed in the fi rst population
to the numbers of the population in the corresponding age
groups of the second. In this way we fi nd the numbers of
deaths that would have occurred in the second population if it
had experienced the mortality rates by age of the fi rst. These
“expected” deaths can be totaled and expressed similarly to
a crude rate by dividing by the total of the second popula-
tion. This comparison is legitimate because the population
base is now identical in its age structure and cannot distortthe results. The process has been called by some “standard-
ization,” and the rates of the fi rst population are described
as having been standardized to the second. Clearly it would
be equally possible to reverse the procedure by standard-
izing the second to the fi rst population. A different pair of
rates would of course be obtained, but it would in general be
found that their ratio was similar to the ratio of the fi rst pair.
An example of the differences of crude and age-adjusted
rates can be observed by using the CDC Wonder system
(CDC, 2004). Table 1 (see previous discussion) shows the
crude and age-adjusted death rates for Parkinson’s disease
(ICD code 332) and cancer of bronchus and lung unspeci-
fi ed (ICD code 162.9). These rates are standardized for 2000
and 1997 for the United States and Pennsylvania. As can be
seen from the table, there is a difference in rates between
crude and age-adjusted as well as for different standardized
populations for the United States and Pennsylvania. This
also illustrates that there are different rates for disease in
specifi c populations, like Pennsylvania versus the United
States. Such rates can be used to evaluate trends for dis-
ease by time and geography. When evaluating and reading
epidemiological studies, it is important to note that the title
of tables and fi gures should fi rst be carefully read so as to
understand the information presented.0025507510012515017520022525020 40 60 80 100Males FemalesMortality rates per 1000AgeFIGURE 1 Mortality rates by age and sex (arithmetical vertical
scale).0 20 40 60 80 100
Age0.1110100250Mortality rates per 1000Males FemalesFIGURE 2 Mortality rates by age and sex (logarithmic vertical
scale).C005_011_r03.indd 372C005_011_r03.indd 372 11/18/2005 10:25:39 AM11/18/2005 10:25:39 AM