Quality assurance program conducted by the USGS in support of the NADP have changed over time. Selected programs have been modified and discontinued after sufficient data were collected to identify bias and variability attributed to sample-collection processes.

Intersite Comparison Program

The intersite-comparison program, which was initiated in 1978 and discontinued in 2004, was designed to assess the accuracy and reliability of field pH and specific-conductance measurements made by site operators. The program was intended to test field-measurement techniques, identify defective equipment, and to describe the performance of NADP/National Trends Network's (NTN) field measurement of pH and specific-conductance. Elimination of the intersite-comparison program was prompted by the discontinuance of field chemistry measurements at NTN sites effective January 4, 2005. NADP members decided that financial resources allocated toward field measurements could be better spent on other aspects of data collection by the network. Specific conductance and pH measurements continue at the Central Analytical Laboratory for all NTN weekly samples.

Sample Handling Evaluation & Blind Audit Program

The Sample-Handling Evaluation (SHE) program was initiated in January 2003, replacing the blind-audit program, which was conducted between 1982-2002. The two programs evaluated the effects that sampling equipment, sample handling, processing, and shipping procedures had on reference solution chemistry to detect bias in weekly precipitation samples collected by the NADP/NTN. Both programs used the paired concentration differences between the sample that was exposed to all handling and processing procedures of a standard weekly sample and a minimally handled sample to assess possible contamination. The programs differed in that samples were submitted to CAL blind, as if they were real samples, for the blind audit, whereas for SHE the CAL knew that the samples were QC samples.

The blind-audit and SHE programs, were discontinued in 2002 and 2004, respectively, because they were determined to be providing information that was not substantially different from the field audit program. See USGS reports for blind audit and SHE data analysis and interpretation. The on-going field audit program, which was initiated in 1997, also uses the paired sample design to asses effects of field exposure, sample handling and processing, and laboratory analysis.