Residential Properties
We have chosen residential properties as the main denominator when creating neighbourhood rates (e.g. crime rates per 1,000 properties) - where a denominator hasn’t already been provided with the data (e.g. the total number of pupils is provided with education attainment data and so used as the denominator in this domain). The number of residential properties is extracted from the Council’s Business and Residential Property Gazetteer.
The main advantages of using residential properties from the Gazetteer are:
It is considered high in data quality standards and meets the British Standard (BS7666) format
It is regularly updated and reflects changes on the ground (e.g. new build, demolition, conversions etc)
Residential properties are identifiable, separately to business properties
Using Geographical Information System (GIS) software, the number of properties can be counted at any given time
Data is available at the neighbourhood level, enabling us to measure the change in number of properties from one time to the next
We have control of the updates so the denominator matches to period of the data
Population as an alternative?
Ideally, the number of people living in an area would be a more suitable denominator in this type of analysis. However, population is not currently used because there are more disadvantages than the use of properties, as given below. (These disadvantages may not apply to other districts, but until authorities undertake a similar exercise, these problems and other similar ones may not surface.)
These disadvantages can be summarised as follows:
The conclusions from this work make it clear that we would be unwise, at this stage, to use population figures as a denominator for our neighbourhoods and indicators. At this stage, properties will continue to be used due to the control, accuracy and flexibility of using the gazetteer database.
The baseline figures
The Office of National Statistics (ONS) produce population estimates for the City. These have been produced, in the light of under-enumeration of the Census, at a City level and at a ward level. The under-enumeration process added population to the inner west wards, but the main undercount of properties was in other areas of the City. Hence, the ward figures for the inner west area appear to be over-counted, the figures for other areas under-counted. This creates a baseline problem when we calculate neighbourhood (as they are mostly sub-areas of wards) population figures.
Change in population estimates
Additionally, ONS change the mid-year population figures, generally in three ways:
Changes to the numbers following additional information that ONS use to produce the figures
ONS consider revising the method they use to population figures, (e.g. the example below, but this is not the only change) and
Following a census, ONS revise the population figures for the preceding decade.
Method of change from one year to the next
When producing the mid-2002 estimates at district level, ONS considered, initially implemented, and then retracted, an annual reduction to the mid-year estimate population figures. For the City, this would have meant a reduction of 500 people per year (0.2%) for 10 years. This reduction arose because the estimate for the year 2000, from 1991, compared with the 2001 estimate would have implied that the methods used to calculate the intervening mid-year estimates overstate the population by 500 per year, or 5,000 over a ten-year period. This presumably arises because ONS undercounts emigrants.
If this proposal had continued it would not have been easy to say from which neighbourhoods these 500 should be deducted. We could not assume that there is a uniform reduction as some areas of the City have a more static population than others.
This suggests that the methods used to calculate population would be unreliable at the small area level.
Growth between years
Following years of a reduction in population size, the City has seen a turn-round in population numbers during the last 5 years.
It is not clear that this growth in population, after allowing for net new housing, a large part of which is flats, has been uniform across the City. We would, therefore, have difficulty allocating any of this growth to our neighbourhoods, especially those where there is evidence of stability.
Hence we would be uncomfortable assuming that there is population growth in some neighbourhoods when there is no evidence of change.
Production of mid-year estimate
The mid-year estimate for 2006 was produced in August 2007. If we were to use the results of this exercise we could not produce our 2006 index until at least September 2007. Whilst we do have problems with accessing data we would be reluctant to produce our previous years' index as late as this. It would be less useful in informing our policies and monitoring change if it was delayed until that time. Whilst this is not an insurmountable problem it does cause a problem for the City.