Evaluation of the use of bias factors with water monitoring data

Abstract Aquatic exposure assessments using surface water quality monitoring data are often challenged by missing extreme concentrations if sampling frequency is less than daily. A bias factor method has been previously proposed to address this concern for peak concentrations, where a bias factor is a multiplicative quantity to upwardly adjust estimates so that the true value is exceeded 95% of the time. In other words, bias factors are statistically protective adjustments. We evaluate this method using a research data set of 69 near‐daily sampled site‐years from the Atrazine Ecological Monitoring Program, dividing the data set into 23 reference and 46 validation site‐years. Bias factors calculated from the reference data set are used to evaluate the method using the validation set for 1) point estimation, 2) interval estimation, and 3) decision‐making. Sampling designs are every 7, 14, 28, and 90 d; and target quantities of assessment interest are the 90th and 95th percentiles and maximum m ‐day rolling averages ( m  = 1, 7, 21, 60, 90). We find that bias factors are poor point estimators in comparison with alternative methods. For interval estimation, average coverage is less than nominal, with coverage at individual site‐years sometimes very low. Positive correlation of bias factors and target quantities, where present, adversely affects method performance. For decision rules or screening, the method typically shows very low false‐negative rates but at the cost of extremely high false‐positive rates. Environ Toxicol Chem 2018;37:1864–1876. © 2018 SETAC