Assessing indicators
Each indicator is composed of one or more measures which changes
over time. Many indicators have a single measure, but where
data cannot logically be combined, such as for the area of
sensitive UK habitats affected by acidity and area affected by
nitrogen, the indicator will have more than one measure. Each
measure is 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 text.
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
year.
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Improving
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Little or no overall change
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Deteriorating
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Insufficient or no comparable data
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For the measures which show trends in
populations of selected species, statistical analysis techniques
have been developed in collaboration with the data providers and
the assessment is based on this analysis. 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 other indicators, 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 standard threshold. 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
a 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.
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
used:
- 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 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.