Indicator use: thresholds and values
To be fully operational, it is advisable that an indicator comes with a simple rule to interpret its significance to an environmental impact. Two different possibilities are proposed, when sufficient knowledge or experience exists on the meaning and the significance of the indicator.
The easier solution is to compare the indicators' result with a threshold, known to correspond to a limit between satisfactory conditions, and unsatisfactory or unacceptable conditions. Some difficulties may arise if the indicator was not tested in adequate conditions and appropriate environment. To ensure the indicators considered in ECASA have been properly tested an extensive field campaign was conducted to field test all the indicators in actual conditions of currently operating production sites of European marine aquaculture.
Another source of difficulty lies with the variability inherent in ecological data, as may be seen in the graph: the threshold of acceptable impact can be reached for different values of the indicator, making it more or less probable. A probability of 0.5 (central curve in black) is generally adopted, but other values can be chosen. Finally the threshold itself is a matter of consensus between social groups, unless precise knowledge can point to a specific value for the threshold.
Another solution to aid interpretation of the information provided by the indicators is to utilize classes, which are assigned a status from very poor to very good. This is the methodology used by the Water Framework Directive. This method allows a more open interpretation of the values taken by an indicator. AMBI is a good example of the use of five classes to characterize indicator values.
Click on the following links for more detail:
ECASA methodology for the selection of indicators
The use of indicators: threshold and value
