Indices are plotted as the blue line on the graphs, and provide a relative measure of population size on an arithmetic scale relative to an arbitrary value of 100 in one of the recent years of the sequence.
If an index value increases from 100 to 200, the population has doubled; if it declines from 100 to 50, it has halved.
For most purposes, the presentation and analysis of longer time-series is required, dating back to before the establishment of the BBS but coming right up to the present day.
The calculation of 25-year alert designations, as in this report, provides just one example.
Bootstrap samples were generated by resampling sites from the original data set, with replacement.
A generalised linear model was then fitted to each bootstrap replicate and a smoothing spline fitted to the annual population indices as described above.
Within a large rectangle of southeastern Britain (covering England and Wales south and east from Seascale, Scarborough and Exeter), the plots are nevertheless believed to be broadly representative, at least of lowland land-classes (Fuller 1985).
This page describes how the results have been combined to derive joint CBC/BBS trends, extending from the 1960s to the present.
As previously noted, the CBC has been an enormously influential project, providing the main source of information on national population levels in the UK since its inception in 1962.
Coverage was predominantly in lowland England, where the numbers of potential volunteers are greatest, while coverage was more patchy in more sparsely populated regions and especially the uplands (Marchant . CBC plots were situated in a limited number of habitats, predominantly farmland and woodland.
Significant differences in trends between the area well covered by the CBC and the rest of the UK were found for approximately half the species (see Freeman . For such species, a regional bias in CBC data means that no reliable UK index can be produced prior to 1994.
In summary, joint population indices dating back to the start of the CBC can continue to be produced for that part of the country well served by the CBC (essentially England) for almost all common species.