Another fundamental factor having an effect on performance level is speech quantity, i.e. the amount of speech material used during the registration phase, and during each user trial phase. Usually, the level of performance increases with speech quantity, but stabilises after a certain amount. As the user's acceptability generally drops when the training phase (and a fortiori the test phase ) lasts too long, a compromise has to be found when the performance improvement is judged marginal.
When registration consists of several training sessions , it is the total training speech quantity, i.e. the speech quantity per training session multiplied by the number of training sessions, which has the clearest impact on performance. When tests are carried out with speech material from several sessions, the relevant factor is the speech quantity per test session .
It may also be important to distinguish the quantity of speech uttered by the speaker from the quantity of speech actually used for training and testing . This distinction is mostly appropriate for event-dependent systems and mainly for diagnostic analysis . In this case, the (average) proportion of speech actually used can be given for information, for the training mode and for the test mode . However, the absolute quantity of used speech should not be reported, in order to avoid ambiguities.
A related factor to speech quantity is speech variety : for a given speech quantity, it is usually more efficient to cover a wider range of linguistic phenomena. Lacking a universal quantitative measure of linguistic coverage, a qualitative description of the linguistic material is the only way of specifying this aspect.