How frequency trends are calculated

These frequency trends suggest whether a species is becoming more or less widespread over time, based on changes in the number of occupied 2km squares and the number of records submitted each year. The calculation uses data from a configured recent period, giving more weight to recent years so that current trends have greater influence on the result. Years with no records are included as zero, where appropriate.

Two trends are calculated for each species:

Distribution trend: how the number of occupied 2km squares changes over time.

Recording trend: how the number of records changes over time.

These two measures are combined, using configurable weights, to produce an overall frequency trend. The calculation checks that enough years contain records and that the trend is sufficiently consistent. Species with too little data are marked as insufficient data, while those with an unclear or unreliable trend are marked as unclear.

Understanding the scores

Scores range from 0 to 100, with the midpoint representing stability:

0–49: Decreasing frequency, with lower scores indicating stronger declines relative to other species at the same taxonomic level.

50: Stable frequency.

51–100: Increasing frequency, with higher scores indicating stronger increases relative to other species at the same taxonomic level.

Species with insufficient data or unclear trends have no score. Species with the same calculated trend can receive the same score. Scores are relative to other taxa of the same taxonomic rank within the same geographical area. They should therefore be compared between species, rather than directly between different taxonomic levels or regions. A frequency trend describes changes in recorded distribution and recording activity. It does not necessarily indicate a change in the actual population size of a species.