United States. General Accounting Office. Social Security Administration: Old-Age, Survivors, and Disability Insurance and Supplemental Security Income for the Aged, Blind, and Disabled; Substantial Gainful Activity Amounts, text, April 23, 1999; (https://digital.library.unt.edu/ark:/67531/metadc293392/m1/1/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .