For indicators within Local Insight, data for custom areas are created in the following way;
For custom areas where an entire Lower Layer Super Output Area (LSOA) is within the boundary of the custom area, the data value for this whole area is included.
For custom areas where part of an LSOA is within the boundary of the custom area, the data is apportioned down to Output Area (OA) level. OAs are included in the custom area if more than 50% of the residential postcodes within that OA reside within the custom area.
This is done in different ways, depending upon what level the data is published to (eg OA or LSOA level):
1. When data is available to OA level: We use the actual OA counts in these cases. For example, this is the case for Census data.
2. Where data is available at below OA level: We aggregate the data to OA level for example for Police.uk and Land Registry data which are published to point (postcode or lat/longitude) level. Where data is a count, data can be summed from the sub-LA geography to get an OA value. Where data is a ratio, score, rate or average value we apply population weighted aggregation using a relevant weighting value e.g. average house prices would be weighted by the number of housing transactions.
3. When data is not available to OA level: As we do not know the exact data value for the OAs, we apportion the published data to OA level. Population data from the census is available down to OA level and we use this data to get a weighting for each OA, by calculating the population of the OA divided by the population of the LSOA (or whatever area-types for which the data is published). We then apply this weighting to approximate the data value for the OAs.
Finally, the data for the custom area is created by summing up the data for the OAs and/ or LSOAs as per above.
Rate/percentage data is created by carrying out the above process separately for both the numerators and denominators and using these to calculate the custom area rate/percentage.
This process is run through our internal databases using MySQL stored procedures to run the aggregation and apportioning process.