They are perceived to be unreliable. Where I differ is in the source of that unreliability. Simply distrusting the integrity of teachers to administer the tests fairly is an affront to honest, hardworking teachers.
Commenting on the concepts of "levelness" I have some sympathy with this being the source of mistrust. Certainly when it comes to the transition from key stage 2 to 3, teachers do not trust that a level 5 at KS2 is the same as a level 5 at KS3.
Put simply, the KS2 level 5 relates to a narrower band of content than the KS3 level 5 and so is different in that respect, but has a comparable level of difficulty.
James D Williams Lecturer in science education Brunel University School of Education 300 St Margaret's Road Twickenham Middlesex