Each of the eleven common parameters were initially compared in bins of one month, in order that any changing trends in the data may be observed. In addition to this, data from the 40 days either side of the March equinox were considered (121 days in all), in order that data from similar seasonal conditions could be compared and finally the comparison was made for the entire data set.
When comparing the occurrence of individual parameters, allowance should be made for times when either or both of the instruments were not working. For example, if the Slough site observed foEs 71% of a month whilst the Chilton site observed foEs only 21% of the month, this means nothing if the Chiton site was not working for half of the time. For the times when both instruments were actually working, the occurrence of each parameter at the two sites was considered. Four percentages are quoted for each period studied;
In addition, the data from times when both instruments were operating were plotted against one another. A correlation coefficient is quoted for this comparison, along with the significance (the number of points is typically so large however, that this figure rarely drops below 100 percent).
Since Slough is the site with which Chilton is being compared, values from this site are plotted along the x-axis for the purposes of this study. To fit a line to these points, the standard errors in Chilton and Slough values were considered to be constant. The gradient and offset of such a line will indicate the presence of any systematic errors between the two instruments. The regression line, along with the scatter of points for each parameter is plotted in the figures labelled 'Chilton versus Slough'.
Finally, the percentage occurrences, gradient offset and correlations as a function of time are presented for each parameter. These plots should demonstrate any annual variation should one exist.