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21 JUN 2019

BCIS Average Prices: but which average?

The BCIS Average Prices section provides a range of statistics for each category. It can be difficult to know which figure should be used for each category.

Average Prices shows three 'averages' and four measures of spread as well as the sample size.

If you are looking for an average figure you can chose from the following.

Mean

This is what most people think of as the average. It is the sum of the rates in the sample divided by the number in the sample. The mean is affected by all rates in the sample and can be unduly influenced by one or two extreme values when the sample size is small. The mean is particularly relevant when you are choosing a rate to be applied across an estate of buildings.

Median

The middle statistic (not the middle of the range). Unlike the mean, this is not as easily affected by rogue figures. It is not the 'typical' rate (see mode) but can reasonably be used as such in most cases. This is probably the best figure to use when choosing a rate for a single typical building.

Mode

The mode is not included in the summary statistics but can be observed as the peak of the histogram. As the most commonly occurring statistic it can be thought of as the typical rate. However, it is not easy to get a reliable figure for the mode, so the median is preferable.

Further details of the statistical terms used in the study can be found in the Frequently Asked Questions section of BCIS Online.