Roberts, Jean. <i>Hearing Levels of Adults by Education, Income, and Occupation, United States, 1960-1962 ; Prevalence Rates for Hearing Threshold in the Better Ear in Excess of 15 Decibels above and 5 Decibels or More below Audiometric Zero, as Determined by Pure-Tone Audiometric Tests at Frequencies of 500, 1000, 2000, 3000, 4000, and 6000 Cycles per Second</i>. U.S. Department of Health, Education, and Welfare, Public Health Services; [for sale by the Supt. of Docs., U.S. Govt. Print. Off.], 1968.