Four Main Steps. Select and group indicators Transform measured data into unit-less scores Aggregate scores from multiple indicators into a summary index Define breakpoints to bin index scores into descriptors of condition (grades or colors) . Central Coast Region Preliminary Indices.
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Magnitude and Exceedance Quotient (MEQ)
Combines two terms that express number of threshold exceedances and magnitude of exceendance.
Derived from Canadian Ministry of Governments method
We are scoring all tests, not just failed tests. This provides a “good” end to the scoring tool
We eliminated the scope term (how many analytes fail) because we have inconsistent data availability across sites.
We do the calculations at the level of the analyte so that each analyte can also be scored
Calculate magnitude (amplitude) for each individual analyte (from CCME)
Calculate unitless sample magnitudes:
Magnitude = concentration/threshold
Calculate normalized sum of magnitudes
NSM = Σ magnitudes / sample count
Scale from 0 – 100
= NSM / (0.01*magnitude average+0.01)
Combine exceedanceand magnitude factors and scale to 100 (modified from CCME)
MEQ = 100 – √exceedance2 + magnitude2
* Note currently no dissolved solids
Factor 1: Scope
Factor 2: Frequency
Factor 3: Amplitude
CCME WQ Index
Quadratic mean (or root mean square) gives greater weight to larger values, thus emphasizing excursions.
Pro:+ widely excepted (esp. for drinking water)
+ considers scope, frequency, amplitude
Con:- resolution to detect differences among sites or times depends heavily on standards selected
- Not as useful for characterizing “healthy” because it only looks at failed tests
- Scope is susceptible to site variability in number of analytes tested (best for very consistent study design)
Calculate rate of exceedance (frequency) for each individual analyte (from CCME)
Factor 1: Frequency