A good measurement is within the interquartile range (IQR), defined as the 25th to 75th percentile or middle half of the measurements. Sulfate and ammonium are good measurements that meet the Data Quality Objectives or DQOs (see Table 1, below). For a measurement within the IQR that fails to meet the DQO, the green hexagon has a gray fill (see potassium).
A satisfactory measurement is outside of the IQR but within the range defined by the median + (IQR/1.349). The ratio, IQR/1.349, is the non-parametric estimate of the standard deviation, sometimes called the pseudo-standard deviation. A measurement that is outside of the median + 1 standard deviation but meets the DQO is an exception to this definition. It is set automatically to satisfactory. Nitrate and chloride are satisfactory measurements that meet the DQOs. When a satisfactory measurement fails to meet the DQO, the green trapezoid has a gray fill (see magnesium).
A marginal or marginally acceptable measurement is outside the range of satisfactory measurements but inside the range defined by the median + 2(IQR/1.349). Marginal measurements fail to meet the DQOs Examples are sodium and calcium.
> Consider a review of sample handling and laboratory procedures <
A biased measurement is outside the range of marginal measurements ( >2 standard deviations from the median). Biased measurements fail to meet the DQOs. Examples are pH and conductivity.
> Corrective actions in sample handling or laboratory procedures may be needed to eliminate biases <
Measurement is below the detection limit of the laboratory’s analytical method. Fluoride is an example.
Measurement was not reported. Acidity is an example.
Measurement | DQO (before January 2018) | DQO (effective January 2018) |
---|---|---|
pH < 4.00 | + 0.07 units | + 0.05 units |
pH: 4.00 - 4.99 | + 0.07 units | + 0.07 units |
pH > 5.00 | + 0.07 units | + 0.10 units |
conductivity | + 7% | + 7% |
acidity | + 25% | + 25% |
sulfate | + 7% | + 5% |
nitrate | + 7% | + 5% |
ammonium | + 7% | + 7% |
fluoride | none | + 20% |
chloride | + 10% | + 10% |
sodium | + 10% | + 10% |
potassium | + 20% | + 20% |
calcium | + 15% | + 15% |
magnesium | + 10% | + 10% |