At most universities and university colleges there are special local collective agreements in place that regulate the salary levels of doctoral candidates who are employed in such positions. Most commonly, these agreements stipulate the lowest, starting salary and then the salary development applicable during the period of employment. This salary development is either linked to period of employment or study achievement.
SULF recommends an increase after a specified period of time and not the study achievement model, partly because difficult assessment situations are avoided which may vary within a university and partially because the study achievement model does not benefit women (due to the fact they tend to take most of the parental leave available) and doctoral candidates who also teach. In addition, salaries are often increased in connection with salary reviews.
Most agreements allow for salaries to be set at a higher level than the minimum and candidates may present special circumstances such as relevant working experience, previous qualifications etc. of the type reported for teachers/researchers above, special educational background or market pressure (i.e. difficulties in recruiting doctoral candidates).
You can find local collective agreements here.