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E. Statistical Methods Used in the Report

Water Quality Compliance Data for Local Authority Areas

In order to present drinking water quality data by local authority area in Section 6, it has been necessary to report data for the group of supply zones within that area. Water supply zone boundaries do not fit local authority boundaries exactly, so the data for any supply zone which falls wholly or partly into the local authority area has been included.

This approach means that data from some supply zones is included twice or more in Section 6. For example, the same data for Alnwickhill B supply zone appear in the sections for East Lothian, Midlothian and City of Edinburgh.

Mean Zonal Compliance

The DWQR used the index known as Mean Zonal Compliance for the first time in the 2005 report. It is a helpful tool when considering water quality at national, regional and local level as it provides a simple means of summarising drinking water compliance and comparing year on year performance. All drinking water quality regulators in the UK are now reporting Mean Zonal Compliance figures using the same methodology, and it should therefore now be possible to make comparisons of this index between the countries of the UK.

Zonal Compliance

Mean zonal compliance for any area is built up from zonal compliance figures for individual parameters in individual supply zones. Zonal compliance is simply the percentage of samples meeting the PCV for that parameter.

Mean Zonal Compliance

The Mean Zonal Compliance ( MZC) for a parameter may be built up for a particular group of supply zones by taking the arithmetic mean of zonal compliances for that parameter across all the supply zones of interest. MZC may be produced for a local authority area, region or for Scotland as a whole in this way.

Overall Compliance

The Overall Compliance for any group of supply zones is the arithmetic mean of the MZCs for every parameter. An Overall Compliance figure for Scotland may be calculated in this way. In 2006, DWQR has used the same parameters in this calculation as the other UK regulators, namely the 40 parameters in Schedule 1 of the 2001 Regulations that have a numerical standard. The full list of parameters may be found in Table 3.4b of this report. In 2007 overall compliance was 99.63%, in 2006 this was 99.44%.

Pesticides

All parameters are weighted equally in the calculation. Scottish Water tests for a large number of different pesticides every year, using a risk assessment process to define sampling requirements in each supply zone. This variation in sampling requirements combined with the sheer number of pesticide determinands has the potential to skew the Overall Compliance calculation by placing undue weight on pesticide analysis. For that reason, results for the individual pesticides not specifically mentioned in Schedule 1 of the Regulations have been pooled to produce a single "All Pesticides" parameter.

Zones with small populations

Some of the water supply zones in Scotland are very small, with populations in single figures. Regulatory sample frequencies are based on population, hence sampling for certain parameters in these zones is infrequent, with perhaps only two samples being taken for each parameter per year. If one of these samples fails, this will adversely affect mean zonal compliance to a much greater extent than a sample failure in a large supply zone. This is unavoidable, and in calculations of regional mean zonal compliance, this effect is compensated for by the large number of these small zones which are present in regions such as the North West.

Distribution Maintenance Index

The Distribution Maintenance Index ( DMI) is the same as the Operational Performance Index ( TIM) used in previous DWQR reports. It is used to reflect the performance of the distribution system for a zone or collection of zones, and is simply the arithmetic mean of the MZCs for turbidity, manganese and iron for that group of zones.

Worked Example

Zonal Compliance

The zonal compliance for iron for a notional supply zone, Zone 1, is calculated as follows:

Mean Zonal Compliance

In order to calculate the MZC for iron for a group of 10 zones which include Zone 1, the arithmetic mean of all the zonal compliances for iron is taken.

Overall Compliance

To calculate overall compliance for the group of 10 zones, the arithmetic mean of the MZC for every parameter is calculated.

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