374 EPIDEMIOLOGY
the 5% level. In many studies a confi dence interval (CI) at
95% is presented. Even if an SMR is above or below 100,
a CI that has an overlap with 100 is often considered to be
in the range of nonsignifi cant. In most cases, statistical sig-
nifi cance exists when the summary value and its CI do not
overlap 100.OCCUPATIONAL MORTALITY COMPARISONSIt will be obvious that precisely the same methods can
be applied to mortality rates from any single disease—or
group of diseases, such as cancer—as to total mortality
from all causes. By appropriate choice of cause groups it
is possible to examine the pattern of mortality in a particu-
lar industry or occupation—for example, to highlight any
excesses or defi cits, when compared to the overall experi-
ence of the total population. But such a comparison often
needs to be made with caution and circumspection; the
total population includes the handicapped, the chronically
sick, and the unemployable, none of whom will be found in
the industrial population. This leads to the healthy- workers
effect (HWE) whereby the overall mortality experience
of the industry is often better than that in the total popula-
tion, partly for the reasons just given and partly because
there may well have been a medical examination to select
only healthy new recruits to the industry. Another effect,
known as the survivor-population effect (SPE) or survivor
effect, arises because those workers in an industry who
fi nd the work too strenuous or beyond their capacity will
leave to fi nd more suitable work; those who remain in the
industry—the survivors—will again be a group selected to
be of better health, stronger, and more competent at the
work. A thorough ongoing epidemiological review of the
industry or of a suffi ciently large factory within it will gen-
erally allow these effects to be separately measured and
assessed, together with the specifi c hazards, if any, that
may be characteristic of the industry.
Many occupational epidemiology studies (McMichael,
1976) now carefully evaluate the infl uence of the HWE and
SPE. Both the HWE and SPE are considered a form of bias.In many ways both the HWE and SPE are similar or the
same occurrence. However, it can be inferred that the SPE
involves, at least initially, those that are best able to tolerate
the work conditions or are best able to cope with exposure
to occupational stress, most notably at the beginning of an
occupational activity. The SPE will likely include the HWE
for those that remain at an occupation for a longer period of
time and would include an adaptive response as would be
related to injuries. Many of the factors associated with these
effects are commonly called confounders. Some of these
would include personal confounders like smoking. Not all
events are equally affected by the HWE. For example, the
HWE has been suggested to have a weak-to-nonextant infl u-
ence on cancer mortality, while having a stronger impact on
mortality from cardiovascular disease (McMichael, 1976).
However, by employing appropriate methodology, con-
founders and the HWE can be controlled for (Mastrangelo
et al., 2004). It should be noted that the most important con-
founders in epidemiology are age, sex, social and economic
status, and smoking, although many others may be important
as well depending on the study. The importance of a con-
founder is best illustrated by cigarette consumption (smok-
ing) and lung cancer (Lee et al., 2001).LIFE TABLESWe have already referred to some of the early essays on the
production of a life table, and to the diffi culties of having to
use various records, because the appropriate mortality rates
were not yet available. When death registration was reason-
ably complete and census suffi ciently accurate, it was possi-
ble to construct a much better life table. William Farr, for his
fi rst life table, used the census of 1841 and the deaths of the
same year. In his second table he broadened his basis, using
both the 1841 and 1851 censuses, and the deaths of a period
of 7 years (1838–1844). Modern practice usually combines
the deaths of 3 years, to reduce the effects of minor epidemic
or climatic variations, and uses the census of the middle
year for the denominators. Mortality rates by sex and single
years of age then enable the construction of a full life table,
advancing in single years from 0 to about 110 years of age.
The successive / x fi gures denote the numbers of living to the
exact age x from the radix at / 0 of 100,000. The larger radix
is justifi ed by the greater degree of accuracy now available.
Essentially the mode of calculation is the same:/x 1 / x d xwhere d x number of deaths between ages x and the day
before attaining age x 1, andd x / x · q xwhere q x mortality rate at exact age x.
Single-year mortality rates are generally obtained as the
ratio of the number in a calendar year of deaths whose age14012010080 60 40 20 0 2040 60 80 10012014001020304050607080Population (thousands)Age (years)Projected population structure with and without the AIDS
epidemic, Botswana - 2020
Males Females
With AIDS
Without AIDSFIGURE 5 Botswana is predicted to have more adults in their
60s and 70s in 20 years’ time than adults in their 40s and 50s.C005_011_r03.indd 374C005_011_r03.indd 374 11/18/2005 10:25:41 AM11/18/2005 10:25:41 AM