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Frequently Asked Question - When “Threshold statistical” is selected in the trend/ threshold matrix display table, why do cells in the indicator matrix sometimes indicate red (above threshold) when the measured concentration at that time point is below the threshold limit?

When “Threshold statistical” is selected in the trend/ threshold matrix display table, why do cells in the indicator matrix sometimes indicate red (above threshold) when the measured concentration at that time point is below the threshold limit?

Cells in the threshold indictor matrix switch colour dependent on the value of the upper 95% confidence limit relative to the user specified threshold concentrations. So, for example, a threshold matrix cell will only switch from red to green when the upper 95% confidence limit of the trend fit is below the user specified threshold concentration.

The user can compare measured concentration data directly against the user specified threshold values by choosing the “Threshold absolute” option in the trend matrix display table. This option is useful for highlighting any one-off measurements that exceed the concentration threshold but are not statistically significant. The user can then determine if it is worthwhile collecting additional data to validate the result. 

CL:AIRE will not be responsible for any loss, however arising, from the use of, or reliance on, the information contained in the responses above, nor do they assume responsibility or liability for errors or omissions in the information provided. Readers are advised to use the information contained herein purely as a guide and to take appropriate professional advice where necessary.