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Diggle’s method
Diggle’s method tests
for a relationship
between an exposure
and a disease, after
correcting for
population density
variation across a
study area. The
method was first
developed
to investigate
larynx cancer cases
that were believed
to cluster around
an incinerator. Here,
we see how it works
using the data from
the original study.
Diggle’s method
predicts the number
of larynx cancer cases
at any point on
the map using 3 pieces
of information.
Firstly, it uses the
density of larynx
cancer over the whole
study area. The more
cancers there are per
unit area, the more
likely you are to find a
cancer case at any
point on the map….
But the chances of
seeing a larynx cancer
case also varies with
population density.
There are likely to be
more cancer cases in
urban areas than rural
areas, simply because
more people live there.
Diggle’s method gets
round this problem by
looking at cases of a
second disease that
occurs randomly. In the
original study, he
identified a disease
which was definitely
NOT linked to the
incinerator: lung cancer
High density areas (like these) are more
likely to have larynx cancer cases than
low density areas.
Cases of this
second disease –
lung cancer – are
used as a measure
of underlying
population density.
A kernel function
(like the Idrisi
FILTER command)
counts lung cancer
cases in different
‘windows’ across
the map. The
result is used as an
estimate of local
population density.
Finally, Diggle’s method also looks at possible point
sources of exposure – in this example, the
incinerator. The method assumes that the risk of
larynx cancer increases, the nearer you live to the
incinerator.
Location of
incinerator
Contour Plot
44000
Legend
raised incidence
Y-coordinate
y
43000
<= 1.000
<= 1.333
<= 1.667
42000
<= 2.000
<= 2.333
41000
<= 2.667
<= 3.000
<= 3.333
40000
<= 3.667
34000
35000
36000
37000
X-coordinate
x
38000
<= 4.000
> 4.000
More on Diggle’s method
• An equation is used to predict how the risk of larynx cancer
increases as you get closer to the incinerator
• This equation is known as a raised incidence model.
• The equation suggested by Diggle is this: 1 + x.e-yD. This is
shown in the previous illustration.
• D represents distance from the incinerator
• The two terms x and y in the equation can be estimated
statistically.
• If x and y are not significantly different from zero, then the
incinerator is not affecting risk of larynx cancer.
• So overall, Diggle’s method tests to see if disease risk
increases near a point source after taking into account
population density
In summary, Diggle’s method predicts the number of disease cases as
follows…
Incidence of larynx cancer at any point
Population density
(measured via
lung cancer)
The method corrects for
population density and
overall rareness of a
disease.
Overall rareness
of larynx cancer
Distance from
incinerator
(point source)
If distance from the point source is
significant, this is evidence that
exposure is increasing disease risk.