The IHR Scoring Algorithm
The Phoenix AMTT uses a scoring algorithm developed by the Institute of Hearing Research, Nottingham. The algorithm was originally used in the Automated McCormick Toy Test device produced by the IHR in the late 1980-1990’s to measure word discrimination thresholds in quiet.
The algorithm works in two phases:
- Homing-in phase: the sound level is reduced in 12 dB steps until the person being tested makes an error. Then the level is raised in 12 dB steps until they respond correctly again.
- Testing phase: after the second reversal, the step size is reduced to 6 dB. The sound levels are presented using a “2 down – 1 up” rule i.e. the person being tested must correctly identify two presentations for the sound level to be reduced by 6 dB, but one failure will result in a 6 dB sound level increase. A reversal occurs where there is an increase or decrease in sound level. The defined threshold used by the IHR is the average sound level over the last six reversals. The test is complete after the sixth reversal.
In this example
Hearing Threshold: 29 dB(a)
Test start level: 68 dB(A)
Initial step size: 12 dB
Final step size: 6 dB
Testing reversals 6
The PHOENIX also uses the original IHR female voice recording, but applies the latest digital technology to produce the sound, and record the test scores.
If speech in noise is selected, then the homing in step size is reduced to 6 dB and the testing step size is reduced to 3 dB.
An initial evaluation of the algorithm and IHR/CHAC Automated McCormick Toy Test can be found in the British Journal of Audiology 1989, 23 245-429, and further evaluation in the British Journal of Audiology 1994, 28, 165-179.