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We might just note that John Snow did a very thorough job - he
managed to get information from 330 out of the 334 households. He
did this by going from house to house to make enquiries. Today,
this type of investigation is often called "shoe-leather epidemiology",
meaning that it involves so much walking that your shoes may wear
out!
In order to estimate the denominator, Snow obtained information
on the number of houses in London whose water was supplied by each
of the two water companies.
Snow noted the source of water in the houses of all those who died
of cholera from 8th July to 26th August 1854, as shown in this table:
| Source of water |
Total no. houses supplied |
No. cholera deaths |
| Southwark and Vauxhall |
40,046 |
1263 |
| Lambeth |
26,107 |
98 |
| Other |
256,423 |
1422 |
Take a few minutes to think about whether there are problems with
using the number of houses supplied by each company as a denominator,
then click below for our interpretation.
 Using
the number of houses as a denominator
However for this example we will assume there was no difference
in the number of people in each house in the areas supplied by the
two water companies.
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