 In their 1993 compendium of historical seismicity in the United States, Carl Stover and Jerry Coffman ignored Richter's revision, and assigned intensities according to their slightly modified interpretation of Wood and Neumann's 1931 scale, effectively creating a new but largely undocumented version of the scale.The basis by which the U.S. Geological Survey (and other agencies) assigns intensities is nominally Wood and Neumann's "Modified Mercalli Intensity Scale of 1931".However, this is generally interpreted with the modifications summarized by Stover and Coffman because in the decades since 1931 it has been found that "some criteria are more reliable than others as indicators of the level of ground shaking".