Historical Data Structure and Laboratory Performance in the North American Proficiency Testing (NAPT) Program.

The North American Proficiency Testing (NAPT) Program is the original PT program for agricultural soil, plant and water testing.  Operated under authority of the Soil Science Society of America, and overseen by society members in academic and industry laboratory testing professions, the program offers a rich tradition of service and depth of expertise that participating labs can rely upon.  One of the key features of the NAPT program has been the large data set obtained over its approximately 20 years of operation.  Hundreds of labs participate each year, providing robust measures for comparative performance evaluation among participating labs.  Moreover, a significant archive of soil samples used in the program has been retained for long-term study and to supply laboratories with check samples for their own QC programs.  Some of the soil samples exchanged with participating labs each year are known as multi-quarter samples and provide additional information on intra- and inter-lab precision over the life of the program.  Two studies currently underway using this large-scale archive of program soils are:  1) evaluation of the long-term accuracy and precision of laboratory performance over the existence of the program for the various test methods evaluated by NAPT, and 2) evaluation of the changes in analyte level for these test methods as a function of the length of sample storage.  These two studies will be reported on and should be of great interest to laboratories in general and participating labs in particular.