Various factors influence the suitability of SRT as an outcome measure. The fundamental problem is that SRT procedures do not adequately constrain the Signal-Noise Ratio (SNR) at which testing takes place. The behaviour of non-linear systems, even simple ones like dynamic range compression, is highly dependent on the SNR at the input; thus the results of comparisons between alternative non-linear (i.e. almost any) hearing-aid systems may be critically affected by:
• Aspects of the SRT procedure itself - corpus, scoring, noise type, acoustics etc., which affect the SNR at which criterion %-correct is achieved
• The inherent SNR needs of the listener, which depend on audiometric loss, cognitive abilities, etc.
Examining past studies on the influence of such factors on SRT, the aggregated range of variation in achievable SRTs is apparently absurdly large. We must therefore conclude that the effects of the various factors are not independent, but subject to, possibly strong, interactions. The issue of ecological validity must also be considered; testing should preferably take place in an SNR range relevant for the user (i.e. realistic), and for the signal processing features under test. In order to address ecological validity, robust documentation of real-life SNRs to provide meaningful constraints on test SNRs is greatly in demand.