Key takeaways
- Unavailable periods are hard constraints.
- Preferred free periods are soft constraints.
- Part-time days off can create clashes across a teaching team.
- If common availability is too narrow, the timetable may become infeasible.
What belongs in hard availability?
Hard availability covers periods when the teacher must not be scheduled. The solver should not place a lesson there.
Preferred free periods are different. They describe a better timetable if the preference can be met, but they should carry a soft penalty rather than block feasibility.
Availability facts to classify
Classify each item before generation.
- Periods when a teacher is unavailable
- Preferred free periods that are negotiable
- Part-time days off
- Team-teaching sessions that need common availability
- Availability limits that affect a whole teaching team
- Requests that should be soft because they are preferences
How to prepare availability constraints
Use one classification pass so the solver receives a clear model.
- List each teacher availability item.
- Mark true unavailable periods as hard.
- Mark preferred free periods as soft.
- Check part-time days off against assigned lessons.
- Check common availability for team-teaching groups.
- Review infeasible results before making requests mandatory.
Why part-time availability can spread through the timetable
A part-time teacher may be available only on certain days or periods. That limit affects every class, team, or shared lesson that needs that teacher.
When two part-time teachers work in one teaching team, the common available time can be much smaller than either person's availability alone. If required lessons cannot fit into that overlap, the hard set may be impossible.
Availability review checklist
Before generation, confirm that unavailable periods are hard, preferred free periods are soft, part-time days off are included, team-teaching sessions have common availability, teacher requests are not all marked mandatory, and infeasible results are checked for narrow availability overlaps.
How availability and soft preferences work together
Availability sets the boundary for feasible placements. Soft preferences then improve the timetable inside that boundary.
This distinction prevents a common modelling error. A teacher who cannot work in a period needs a hard constraint. A teacher who would rather be free in a period needs a soft constraint.
Questions planners ask about teacher availability
What is the difference between unavailable and preferred free?
Unavailable means never schedule the teacher in that period. Preferred free means avoid the period if possible, with a soft penalty if it cannot be avoided.
Why do part-time teachers create wider clashes?
Their limited availability can transfer to the teaching team around them. Two part-time teachers in one team can teach together only at times when both are available.
Should all teacher requests be mandatory?
No. Mandatory unavailable periods should be hard. Requests that express a preference should be soft, otherwise the model can become over-constrained.