Subjective tests are carried out to assess the quality of an entity as perceived by a user. However, several characteristics inherent to the subject or to the test methodology might influence the user's judgments. As a result, reference conditions are usually included in subjective tests. In the field of quality of transmitted speech, reference conditions correspond to a speech sample impaired by a known amount of degradation. In this paper, several kinds of reference conditions and the process used for their production are presented. Examples of the corresponding normalization procedure for each kind of reference are given.