Operating Instruction
ERO•SCAN
®
Screening and Diagnostic Version
operating_instructions_8106556-6_EROSCAN_e_15a
60
8106556-5 02/15
Appendix C: Pass/Refer Criteria
Pass/Refer Criteria for DPOAE
The decision that a DPOAE exists is based on detecting a signal whose level is significantly
above the background noise level. This requires a statistical decision, since the random noise
level in the DPOAE filter channel can be expected to exceed the average of the random
noise levels in the four adjacent filter channels — used as the reference for comparison —
roughly half the time.
Extended measurements of the noise distributions in both the DPOAE filter channel “DP
level” and the rms average of the 4 adjacent channels “N level” indicate that the signal-to-
noise ratio (the difference between DP and N) has a standard deviation of 5.5 dB. As shown
in Diagram 1 (page 60), this implies a 10 % probability of seeing a 7 dB SNR simply from
the variability of the noise levels in the 2 filter sets.
Requiring an SNR of 6 dB in three out of four frequencies drops the probability of passing
an ear with significant hearing loss to 1 % or less.
NOTE: By the binomial distribution, two of three frequencies at >8.4 dB or three of six
frequencies at >7 dB should also ensure less than 1 % probability of passing a moderately-
severe hearing-impaired infant.
Preliminary ERO•SCAN
®
trials with infants indicate that the tester’s technique is the single
most important variable in the pass rate on normal-hearing infants. Some testers pick up the
technique (see
Operating Instructions section, page 18) with only a couple of day’s practice,
producing pass rates comparable to those for other DPOAE equipment they have used for
months; other testers take longer.
Occasional claims of extraordinarily low probabilities of missing an ear with hearing loss
appear to be based on poor statistics. As discussed by Gorga (Mayo Clinic Teleconference,
1998), since the incidence of significant hearing loss is roughly 2 per 1,000, verifying a 99.7
% accuracy would require testing hundreds of thousands of babies with a given system.
Thus to demonstrate that only 3 babies out of 1,000 with hearing loss were missed would
require follow-up testing on 500,000 babies. To our knowledge, no one has performed
such tests to date.
Summary of Contents for ERO SCAN
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