Summary:
Maps
& Stats allows you to extract data for Bradford District
and areas within it. Bradford Council have used data supplied as tables for
small areas, such as census Enumeration Districts, electoral
Polling Districts, and postal areas. For consistency, each of
these datasets has been spread to postcode points within
Bradford District.
The resulting 'boundary-free' data
allows Maps & Stats' flexible approach to reports.
In More Detail:
Three types of area are
important when providing you with statistics:
- Data Units: the areas for which the data were
originally supplied. These may be 1991 Census enumeration
districts, electoral polling districts or wards, postal
sector areas, or postal code units. The data units are
different for each report, and are specified on each report.
These original data are not available to you.
- Postcode Points: the points to which the data
have been spread for transfer to Maps & Stats. There are
fifteen thousand postcode points in the Bradford District,
each based on the centre of a unit postcode. Each value for
a data unit has been disaggregated (spread) to the postcode
points within it, in proportion to the number of residential
addresses that belong to those postcodes.
- Report Units: the areas for which data are
re-aggregated for presentation to you. When you choose a
report for a standard area (for example a Ward or a
Neighbourhood area) or when you draw your own area boundary,
the data for all the postcode points that lie within your
chosen area are added together to provide the report you
have requested. Reports for the standard areas have been
prepared prior to your entry onto the site, to save time
when you request statistics for these areas.
For
maximum accuracy, where the original data is available for
report units, it is used directly without the intermediate use
of postcode points. For example this is the case with census
and population estimates when reported for electoral
wards.
In other cases, your report will not contain
exact figures but will be a very good estimate for that area.
It will use only the figures from each data unit that your
area overlaps. It will take a sensible proportion of those
data units that your areas does not wholly include. In
general, the error in the reports is smallest when the report
unit that you choose is larger than the original data units
that are specified on the report.
Modification of
postcode data to maintain confidentialitySome data
were supplied for each postcode unit (eg. BD3 0JZ). In this
case, and before introduction into the web site, all data for
postcodes with a small number of residential addresses are
aggregated together within their local census enumeration
district. They are then disaggregated or spread as described
above. This modification maintains the accuracy of the reports
for local areas, but ensures confidentiality.
Top of
PageThresholdsWhen choosing to draw
your own area, a report will not be presented unless your area
contains at least 150 residential addresses. This threshold is
set to the approximate number of residential addresses in a
census Enumeration District. The use of thresholds is to
discourage the calculation of reports of data that may not be
reliable for smaller areas.
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