A note on data quality control and data screening.

This website contains data for approximately 88,000 water samples collected beginning in 1974. We cannot guarantee that each data point is free from sampling bias/error, analytical errors, or transcription errors. However, since its beginnings, the WQL has operated a substantial internal quality control program and has participated in numerous external quality control reviews and sample exchange programs. These programs have consistently demonstrated that data produced by the WQL is of high quality.

The data provided in the RiverData files have all been screened by WQL staff. The purpose of the screening is to remove outliers that staff deem likely to reflect sampling or analytical errors rather than outliers that reflect the real variability in stream chemistry. Often, in the screening process, the causes of the outlier values can be determined and appropriate screening actions taken. These may involve no change in the sample values, alteration of the values or simple deletion of those data points.