What do these rates mean?
As we said before, to interpret
these rates we need to compare them with the expected rates derived from
elsewhere. There are all sorts of ways we can compare rates.
Below we give the observed and the expected number of cases and calculate
the probability that the observed number (or an even higher number) of
cases might occur simply by chance if the true rate in the Pembroke Road
area were the same as elsewhere. The relative
risks are also given.
These figures are calculated
for the period 19851994 when there was a peak incidence in cases of leukaemia,
that is to say, we have taken the "worst possible case scenario".
Childhood Leukaemia in NN5 19851994:

Annual rate per million children under 15 = 107.8

Number of cases observed = 6

Expected annual rate per million children under 15 = 40.5^{¶}

Average number of cases expected = 2

The probability of seeing six or more cases by chance if the true rate
is the same as the national rate is about 1 in 40

Relative Risk = 2.7
Childhood Leukaemia in NN5 7** 19851994:

Annual rate per million children under 15 = 225.5

Number of cases observed = 5

Expected annual rate per million children under 15 = 40.5^{¶}

Average number of cases expected = 1

The probability of seeing five or more cases by chance if the true rate
is the same as the national rate is about 1 in 400

Relative Risk = 5.6
^{¶ NB This figure is the crude rate/million children under
15 for Northamptonshire for the same period (national figures are not yet
available but these will be very similar). }
Another way of looking at the
figures for NN5 7**, is that we would expect there to be at least 15 clusters
of childhood leukaemia in the UK with the same or a larger number of cases
of childhood leukaemia as seen in the Pembroke Road area (where there are
the same number of children we have) purely due to chance ^{6}.
When interpreting these figures
it must also be remembered that the calculated probability of a particular
cluster occurring (if the true rate is the same as the national rate)
is much smaller that the true probability of any cluster of this size or
larger having occurred. This is explained in section 5.2
below.
All Childhood Cancers
Childhood cancers other than
leukaemia are not higher than expected in the Pembroke Road area..
The annual rate for all childhood
cancers for the period 1962 1994 is 125 per million children under 15
for NN5 and 150 per million children under 15 for NN5 7**. The national
annual rate for all childhood cancers is 115 per million children under
15. At first sight this might suggest that other childhood cancers
are also higher than expected in these areas. However, this is not
true. The excess is simply due to the raised leukaemia rates (as
we noted above leukaemia accounts for over 1/3 of childhood cancers).
The rate for all childhood cancers other than leukaemia in NN5 is 76 per
million children under 15 which is exactly the same as the national rate
^{5}. The figure for NN5 7** is
82 per million children under 15. This is not significantly different
from the national rate. 