. The Bell System technical journal . ge hearing acuitysufficient to justify the expectation of associating the departure with aspecific cause. In Fig. 9 there is shown a typical distribution curve for hearing shows the relative frequency of occurrence of various degrees ofhearing acuity among a given class of people. Such curves can bewell described for many practical purposes by two quantities, theaverage hearing loss and the standard deviation. The latter quantity,designated as cr, is a measure of the spread of the individual valuesfrom the average. The experience of statisticians w
. The Bell System technical journal . ge hearing acuitysufficient to justify the expectation of associating the departure with aspecific cause. In Fig. 9 there is shown a typical distribution curve for hearing shows the relative frequency of occurrence of various degrees ofhearing acuity among a given class of people. Such curves can bewell described for many practical purposes by two quantities, theaverage hearing loss and the standard deviation. The latter quantity,designated as cr, is a measure of the spread of the individual valuesfrom the average. The experience of statisticians with distributions of observations ofwidely different character indicates that there is little chance of assign-ing specific causes for the deviations of observations which lie closer ^ E. G. Witting and W. Hughson, Inherent Accuracy of Repeated ClinicalAudiograms, Laryngoscope, 50: 259 (1940). ^^ W. C. Beasley, Correlation Between Hearing Loss Measurements, Jour,Acous, Soc. Amer., 12: 104-113 (1940), 560 BELL SYSTEM TECHNICAL JOURNAL. MEAN HEARING LOSS Fig. 9—Typical distribution curve of hearing loss. than 2(T to the average, because, in this range, many causes are operat-ing and none is predominant. In the range from 2a to Z(x, the dottedarea of Fig. 9, there is a good chance of isolating causes, although theattempt might not be justified if it involved great discomfort, dangeror expense. Beyond 3a, the cross-hatched area of Fig. 9, there is anexcellent chance that the cause, or causes, can be isolated.^^ Thisdoes not imply that findable causes of hearing impairment do notexist in Individuals lying closer to the average than these limits, butmerely that the hearing test is not useful in selecting them. Theidentification of such causes and their treatment are, of course,medical problems. TABLE 16Hearing Loss at Which Deafness Begins, in the Sense Given in the Text Limit Frequency 440, 880 1760 3520 7040 2<r BoysGirls BoysGirls 1414 2121 15 13 2220 1911 2918 2215 3424 This
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