AS Science In Society 1.1
Teacher notes
Introduction
This activity extends the textbook account of John Snow’s work with a
description of his epidemiological work in South London, including
quotes from Snow himself. It encourages group discussion of his
methods and some of the data he collected.
The activity
The questions are meant as teaching points, not as tests of
comprehension
One way to use them is to get students to briefly discuss their answer to
each question in pairs; then to have a teacher-led discussion with the
whole class. It should take about 30 minutes.
Suggested answers to the questions
1. What correlation is shown by the data?
A correlation between the source of water and the death rate. The
houses who took water from the Southwark company had a higher
death rate than those who used Lambeth water.
2. Do the data support Snow’s hypothesis, that cholera is caused by an
infectious agent carried in water?
Yes they do. There is a very strong correlation between water supply
and deaths, which is exactly what Snow’s hypothesis would predict.
When a prediction is confirmed by
observation it provides strong support for the hypothesis which led to
the prediction.
References
Textbook Chapter 1
Specification 3.1.1
Further information on John
Snow from:
http://www.ph.ucla.edu/epi/sno
w.html
How Science Works
Ba If something happens only
when a factor is present, we say
there is a correlation between the
factor and the outcome. If one
variable increases (or decreases)
steadily in value as the value of
another variable increases, this is
even stronger evidence that the
two variables are correlated.
Bd To investigate the relationship
between a factor and an outcome,
it is important to control all other
factors that might affect the
outcome. Then changes in the
outcome can be attributed to the
factor that is being changed.
3. Do the data prove Snow’s hypothesis?
Cd Scientists test an explanation
No they does not, for two reasons:
by seeing if specific predictions
based upon it are in agreement
• There is a correlation but this could be caused by some other factor
with data from observation or from
which we do not know about which was different in these particular
an experiment (a deliberate
streets - such as the age of people or the level of
intervention to generate data).
overcrowding.
• A correlation never provides definite proof. A causative mechanism
must be found. This only happened in 1883 when Koch discovered the bacterium which causes
cholera.
4. Snow's results, shown in the table, are not easily explained by the miasma theory. Why not?
If miasma were the cause of cholera we would expect the correlation to be between the street lived
in and deaths, because everyone in the same street would have to breathe the same air
with the same smells.
5. Why did Snow think the social class and age of the people he studied was worth mentioning?
Social class or age could be variables which affected the death rate from disease. The fact that his
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AS Science In Society 1.1
Teacher notes
sample included all classes and ages randomly in both groups excludes this as an explanation.
6. Why is the column "deaths per 10 000 households" is the most useful for comparison?
Because the two groups are not the same size, the total number of deaths would be different even if
water supply had no effect. This gives a ratio, used in the same way as a percentage where
percentages would give very small fractional numbers. A ratio or percentage allows comparisons to
be made between two samples of different size.
7. Why did earlier attempts to reduce cholera, based on the miasma theory, in fact increase the number
of cases in customers of the Southwark water company?
In an attempt to reduce smells, the human waste (including any cholera bacteria) was pumped into
the Thames and then into other peoples’ houses by the Southwark water company. The wrong theory
can do real harm.
December 2007
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©The Nuffield Foundation, 2007
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AS Science In Society 1.1
Student sheets
By 1849 John Snow had developed the hypothesis that cholera was transmitted in contaminated water.
However his evidence could also have been explained in other ways. Few people were convinced by
the new hypothesis.
When the next cholera epidemic broke out in 1854 Snow was able to plan an investigation that he hoped
would show a correlation between contaminated water and cholera. He undertook painstaking house to
house surveys, investigating the source of water used by cholera victims. In one region in South
London some households had water piped from the Lambeth water company whilst others in the same
streets were supplied by the Southwark company.
 The Lambeth company took its water from a clean stretch of the Thames before it entered London.
 The Southwark company used polluted Thames water in London.
John Snow described his work as follows:
No fewer than 300 000 people of both sexes, of every age and occupation and every rank and
station, from gentle folks down to the very poor, were divided into two groups without their choice,
and in most cases without their knowledge; one group being supplied with water containing the
sewage of London and amongst it, whatever might have come from the cholera patients, the other
group having water quite free from such impurity. To turn this grand experiment into account all
that was required was to learn the supply of water to each individual house, where a fatal attack of
cholera may occur. (Snow 1855)
Water company
Number of
households
Cholera
deaths
Southwark (water
from Thames in
London)
Lambeth (clean
water from above
London)
40 046
1263
Deaths per
10 000
households
315
26 107
98
37
This research is an early example of epidemiology, the investigation of the causes of disease using
surveys of the possible factors affecting the health of a population.
1. What correlation is shown by the data?
2. Do the data support Snow’s hypothesis, that cholera is caused by an infectious agent carried in
water?
3. Do the data prove Snow’s hypothesis?
4. Snow's results, shown in the table, are not easily explained by the miasma theory. Why not?
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©The Nuffield Foundation, 2007
Copies may be made for UK in schools and colleges
AS Science In Society 1.1
Student sheets
5. Why did Snow think the social class and age of the people he studied was worth mentioning?
6. Why is the column "deaths per 10 000 households" is the most useful for comparison?
7. Why did earlier attempts to reduce cholera, based on the miasma theory, in fact increase the number
of cases in customers of the Southwark water company?
Homework
8. Snow himself campaigned actively to get the authorities to apply his new theory to the prevention of
cholera. Write a short account of the research described above, suitable for a newspaper of the time,
with a headline, which would have helped the campaign. Not more than 350 words.
Further information on John Snow from http://www.ph.ucla.edu/epi/snow.html
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©The Nuffield Foundation, 2007
Copies may be made for UK in schools and colleges