2.3. Statistical analysis
Daily pollutant concentrations for the two-year period before
and the two-year period after the scheme’s introduction on the
17th February 2003 were summarised using geometric means.
Changes in pollutant geometric mean concentrations pre- and
post-CCS implementation were expressed as percentage changes.
Regional and trans-boundary sources of all pollutants excluding
ozone did not change significantly over the 2-year pre- and postintervention
periods (data not shown). They therefore had no
impact upon the calculation of the relative changes in pollution.
However, rural concentrations of ozone increased by 1.7 ppb
between the pre- and post-CCS periods. The calculation of the
percentage changes in ozone concentrations after the implementation
of the CCS were therefore adjusted for this absolute
increase across the south east of England.
The software package SPLUS was used for all analyses.