370 EPIDEMIOLOGY
different sources of exposure (Paneth, 2004). The population
with a low number of deaths was obtaining water from a
brewery source that had its own well, which as we now know
was not contaminated, and those in the second population,
having a high number of deaths, were obtaining it from the
Broad Street pump. From these data, he plotted the occur-
rences and extent of the outbreak, which we now look at as
the duration of the epidemic. Near the end of the epidemic,
Snow had the Broad Street pump handle removed for to pre-
vent the reoccurrence of the disease. From his investigation,
a foundation of causative agents (which was not known at
the time), population characteristics, environment, and time
were connected in evaluating the disease process with an
applicability of prevention.
During an epidemic in 1853, Snow examined the sources
of water. At the time, there were three water companies serv-
ing the area, Southwark, Vanxhall, and Lambeth. Southwark
and Vanxhall collected water from a polluted section of the
Thames river, while Lambeth collected water upstream of the
pollution. By using deaths published by the registrer general
in London, Snow was able to deduce that those obtaining
their water from Southwark and Vanxhall had a much higher
death rate that those getting water from Lambeth. Snow
obtained the addresses of those that died, and by knowing
the water source and the population in the area, he was able
to calculate death rates for the various water sources. He
determined that those having Southwark and Vanxhall water
experienced a death rate of 315 per 10,000 and those with
Lambeth had a rate of 37 per 10,000 (Lilienfeld and Stolley,
1994). This provided evidence that obtaining water from the
polluted area of the river resulted in a high rate of death from
cholera and that cholera was a waterborne disease. Snow’s
discovery, through epidemiology, occurred approximately
40 years before Robert Koch, in 1884, identifi ed Vibrio
cholera as the causative agent of cholera. This certainly
established a relationship of disease with the environment,
but also showed the importance of representing epidemio-
logical data in the form of a rate. Today, rates are commonly
reported as a number per 100,000 or million. However, any
rate expression is acceptable. Even when cause of a disease
is not known, as shown by Snow, a great deal can be learned
about the agent through epidemiology.
Today, the pump handle from the Broad Street well is in
possession of the John Snow Society. One survey reported
that Snow was the most infl uential person in medicine, with
Hippocrates being second (Royal Institute of Public Health,
2004). Certainly this report does suggest that there may be
bias in the survey, with a larger number of votes coming from
the John Snow Society, but it illustrates the importance of his
contribution and the infl uence that epidemiology has had on
medicine. It should be mentioned that Snow was one of the
originators of the fi eld of anesthesiology as well. Thus, his
contribution is not limited to pure epidemiology.
From these examples, it becomes clear that the meth-
ods of epidemiology are in essence those of statistics and
probability. It is also clear that much of medicine is based
on observation within the fi eld of epidemiology; diagnosis
depends upon a recognizable cluster of signs and symptomscharacteristic of a disease, but this is only so because of
their statistical similarity extended over many cases. And in
a like manner, the appropriateness and effi ciency of treat-
ment methods summarize the result of practice and observa-
tion. Many of the developments of modern medicine, both in
methods of diagnosis and treatment, depend upon epidemio-
logical procedures for their assessment and evaluation, such
as in clinical trials (see below) and many experimental studies,
as was illustrated by Dr. Snow’s study of water sources.
Generally, epidemiological studies can be divided into
four groups: ecological, cross-sectional, case-control, and
cohort. Ecological and cross-sectional studies are hypothesis-
generating investigations, while case-control and cohort
studies can establish a causal effect. Case-control and cohort
studies can provide odds ratios (ORs) and relative risks
(RRs). In most cases, the OR and RR will be equal to each
other, and represent the risk associated with exposure and
occurrence of disease.MORTALITY AND THE FIRST LIFE TABLESIt is in the description and measurements of mortality that
we fi rst meet quantitative epidemiology. The London Weekly
Bills of Mortality begun early in the sixteenth century con-
tinued irregularly during that century and were resumed in
1603, largely to give information about the plague. John
Graunt published an analysis and comparison of them in
the middle of the seventeenth century ( Natural and Political
Observations upon the Bills of Mortality ), and later Sir
William Petty published Five Essays in Political Arithmetic ,
a book that was devoted rather less to numerical data that
was Graunt’s. Graunt had examined deaths by causes and
age, which led to the interest at this time in the construc-
tion of life tables. A life table aims to show the impact of
mortality by age through a lifetime. Starting with an arbi-
trary number of people (e.g., 1,000—known as the “radix”)
who are regarded as having been born at the same time, the
life table thus opens with 1,000 persons at exactly age zero.
A year later this number will be diminished by the number
of infant deaths that have occurred among them, leaving
as survivors to their fi rst birthday a number usually desig-
nated 1. Similarly, the deaths occurring in the second year
of life reduces the number still further, to 2. By the same
process the diminution of numbers still alive continues until
the age at which none survive. The fi rst actual life table was
constructed in 1693 by Edmund Halley, the mathematician
(best known perhaps for the comet named after him), and it
was based on 5 years’ experience of deaths in the German
city of Breslau. Since it recorded deaths by age, without ref-
erence to birth, the radix was obtained from a summation
that the population was in dynamic equilibrium. Although
there were other life tables constructed around this time,
when life-insurance companies began to be founded, it
was not possible to construct an accurate life table without
using rates of mortality rather than numbers of deaths. Rates
required denominators to be both appropriate and accurate,
and the obvious source was a census.C005_011_r03.indd 370C005_011_r03.indd 370 11/18/2005 10:25:39 AM11/18/2005 10:25:39 AM