While the general lines of the reduction scale are similar among different authors, there is no consensus concerning the exact order in the hierarchy. Consider the following reduction hierarchies: the higher an element is in the scale, the more likely it is that it will be reduced in an unstressed syllable. While Kager (1989) and Van Oostendorp (1995) focus on A-class vowels, Booij (1995) also incorporates some B-class vowels in the scale:

As can be observed, the most crucial difference among the different proposals is to be found in the classification of /ø/, which Kager and Van Oostendorp regard as being in the middle of the hierarchy (together with /o/), whereas Booij groups it with the high vowels /u, y/. These differences can be attributed to the limited number of relevant data, as /ø/ rarely occurs in unstressed syllables at all. Diphthongs never reduce, therefore, they are absent in the table.