Water Resources Division, U.S. Geological Survey 1995 Every year in the United States, millions of measurements are made on the chemical composition of items that affect us on a daily basis. Determining the accuracy of these measurements is based on the analysis of appropriate reference materials whose composition was previously determined through rigorous testing. Today, reference materials help us evaluate the composition of the food we eat, medicine we use, soil we grow our crops in, and hundreds of other products that affect our everyday lives. application/pdf 10.3133/fs20795 en U.S. Geological Survey USGS reference materials reports