Each indicator is composed of one or more measures which will
show trends over time. Many indicators have a single measure,
but where data cannot be combined logically, the indicator will
have more than one measure. Each measure is summarised or
assessed separately using a set of ‘traffic lights’. The
traffic lights show ‘change over time’. They do not show
whether the measure has reached any published or implied targets,
or indeed whether the status is ‘good’ or ‘bad’, although where
targets have been set, these are identified in the indicator
The traffic lights are determined by identifying the period over
which the change is to be assessed and comparing the value of the
measure in the base or start year with the value in the end
Little or no overall change
Insufficient or no comparable data
Where possible the assessment has been made by evaluating trends
using statistical analysis techniques. The assessment may be
made by Defra statisticians in collaboration with the data
providers, or undertaken by the data providers themselves. A
green or red traffic light is only applied when there is sufficient
confidence that the change is statistically significant and not
simply a product of random fluctuations.
For some indicators, it is not possible to formally determine
statistical significance and in such cases the assessment has been
made by comparing the difference between the value of the measure
in the base or start year and the value in the end year
against a ‘rule of thumb’ threshold. The standard threshold
used is three per cent, unless noted otherwise. Where the
data allow it, a three-year average is used to calculate the
base year, to reduce the likelihood of any unusual year(s) unduly
influencing the assessment. Where an indicator value has
changed by less than the threshold of three per cent, the
traffic light has been set at amber. The choice of three per
cent as the threshold is arbitrary but is commonly used
across other Government indicators, and use of this approach
is kept under review.
The traffic lights only reflect the overall change in the
measure from the base to latest year and do not reflect
fluctuations during the intervening years.
Where data are available, two assessment periods have been
- Long-term – an assessment of change since the earliest date for
which data are available, although if data do not precede 1996 a
long-term assessment is not made.
- Short-term – an assessment of change since 2000 (or the closest
date for which data are available).
The UK Biodiversity Indicator Steering Group, which
reports to senior civil servants in the four countries, is
considering adopting a ten year rolling period for the short-term
change assessment, which will mean that when 2020 is reached, the
assessment will be over the period of the
Strategic Plan for Biodiversity 2011–2020.
The individual indicators also have a third marker showing
the direction of change in the last year. This period is too
short for a meaningful assessment. However, when it exceeds a
one per cent threshold, the direction of change is given
simply as an acknowledgement of very recent trends and as a
possible early warning of emerging trends.