The great majority of speech output assessment techniques use listening experiments involving human subjects, i.e. functional and/or judgment tests of speech output at the acoustic level. In the following subsections we will discuss a number of methodological issues that are relevant especially to this type of testing. The issues concern the choice of subjects, test procedures , benchmarks and reference conditions , and precautions to ensure cross-language comparability . Although there is no a priori reason why this should be so, no accepted methodology seems to exist for other types of speech output evaluation techniques. As will be obvious from later sections, for example, no accepted methodology can be identified in the field of output evaluation at the symbolic linguistic level. It is unclear in this area what kinds of textual materials should be used in tests, what error categories should be distinguished, and what scoring procedures should be used. We will therefore limit the methodological discussion to acoustic output testing techniques involving human listeners.