Hearing levels of youths 12-17 years, United States : distribution of monaural hearing levels of youths 12-17 years at frequencies of 250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 hertz as determined in individual air-conduction tests, by age, sex, race, geographic region, size of place of residence, family income, and education of parent / - Roberts, Jean