Interacoustics OtoRead Instructions For Use Manual page 62

OtoRead™ Instructions for Use - US Page 56
Appendix B: test sequence
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 the Diagram below, 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.
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.
TM
Preliminary OtoRead 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 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.