4. Test method
To conduct subjective assessments in the case of systems generating small impairments, it is
necessary to select an appropriate method. The “double-blind triple-stimulus with hidden reference”
method has been found to be especially sensitive, stable and to permit accurate detection of small impairments. Therefore, it should be used for this kind of test.
In the preferred and most sensitive form of this method, one subject at a time is involved and the
selection of one of three stimuli (“A”, “B”, “C”) is at the discretion of this subject. The known
reference is always available as stimulus “A”. The hidden reference and the object are
simultaneously available but are “randomly” assigned to “B” and “C”, depending on the trial.
The subject is asked to assess the impairments on “B” compared to “A”, and “C” compared to “A”,
according to the continuous five-grade impairment scale. One of the stimuli, “B” or “C”, should be
indiscernible from stimulus “A”; the other one may reveal impairments. Any perceived differences
between the reference and the other stimuli must be interpreted as an impairment.