Assessing indicators

Many indicators have a single measure which changes over time 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,using a set of ‘traffic lights’. The traffic lights assess ‘change over time’.They do not assess 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.

 

	improving
	little or no overall change
	deteriorating
	insufficient or no comparable data

 

The traffic lights are determined byidentifying 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.

 

For the measures within the indicators on 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 ‘significant’ and not simply a product of random fluctuations.

 

For other indicators, the assessment has been made by comparing the value of the measure in the base or start year with the value in the end year against a standard threshold. 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 3 per cent, the traffic light has been set at amber. The choice of 3 per cent as the threshold is arbitrary but has been used successfully in 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:

 

1. 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.
2. 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 the direction of change is given simply as an acknowledgement of very recent trends and as a possible early warning of emerging trends.