Constraint recipe
Spread means distributing a subject's weekly lessons across several days instead of clustering them. Three maths lessons on Tuesday is clustered; one each on Monday, Wednesday, and Friday is spread. Schools usually treat spread as a soft preference, sometimes called a distribution preference, because blocks, part-time teachers, and scarce rooms can make an even pattern impossible.
Key takeaways
- Spread is about which days a subject uses, not how many lessons it has.
- Double periods and workshop blocks cluster lessons on purpose.
- Part-time teachers and scarce rooms can make even spread impossible.
- Tighten spread only after clashes and required lessons are valid.
What spread controls
Spreading a subject across the week is one of the standard quality preferences in school timetabling, alongside gap limits and morning placement for core subjects. The number of required lessons stays fixed; only the day pattern changes.
Spread is usually a soft rule: the generator pays a penalty for each extra same-day lesson and avoids clustering where it can. Making spread hard is risky, because one part-time teacher or one scarce room can leave clustering as the only valid outcome, and a hard rule then blocks the whole timetable.
A worked example: three English lessons, two patterns
Class 8A has three English lessons per week. In the first draft they land on two days, with a same-day pair on Monday. The spread pattern uses three days instead.
Before: clustered on Monday and Tuesday
| Mon | Tue | Wed | Thu | Fri | |
|---|---|---|---|---|---|
| P1 | Eng | ||||
| P2 | Eng | ||||
| P3 | |||||
| P4 | |||||
| P5 | Eng |
- Clustered lessons
After: one lesson on each of three days
| Mon | Tue | Wed | Thu | Fri | |
|---|---|---|---|---|---|
| P1 | Eng | ||||
| P2 | Eng | ||||
| P3 | Eng | ||||
| P4 | |||||
| P5 |
- Spread lessons
Now the catch: if the English teacher works only Monday and Tuesday, the after picture is impossible without restaffing. The draft was not a generator failure; it was the only legal shape. This is the first thing to check when a subject stays clustered after you raise the spread preference.
When clustering is the right answer
Some clustering is designed, not broken. Double periods and lab blocks put two lessons together because the activity needs the time. Vocational programmes often alternate theory and practical blocks, and an intensive week concentrates one subject on purpose.
Exempt those from spread rules instead of fighting them. See soft constraints before you consider turning spread into a hard rule.
Tune spread without breaking the week
- List the subjects that should touch more than one day, with a minimum day count for each.
- Exempt mandatory blocks, labs, and double periods from the spread rule.
- For each clustered subject, check the teacher's working days and the eligible rooms' free days. If either is two days, so is the subject.
- Set or raise the spread preference and regenerate.
- Compare the result against teacher gaps and daily balance. More spread often means more teacher movement.
- Record the subjects where availability explains the remaining clustering, so next year's staffing can fix what this year's timetable cannot.
Spread mistakes
- Forcing spread on subjects that intentionally run as blocks
- Raising the spread weight when the real limit is a two-day teacher
- Trading a spread win for worse teacher gaps without checking either count
In Smootables
Spread is the Subject Spread preference: open Settings, then Timetabling, and raise Subject Spread (0 to 200). Zero, the default, turns it off. The description in the app reads: avoid scheduling the same subject multiple times per day, higher means more spread. Each extra same-subject lesson a group has on one day costs the generator a penalty, so lessons drift onto more days where availability allows.
It is a preference, not a rule: clashes, availability, and required lessons always win. After generation, the analysis summary reports a Study unit spread score, so you can compare two runs before committing to a heavier weight.
- Raise Subject Spread in steps (the app's guide: 50 low, 100 medium, 200 high) rather than jumping to the maximum.
- Compare Study unit spread and Teacher gaps across runs; more spread often costs compactness.
Quick answers
Is lesson spread hard or soft?
Usually soft. The number of required lessons is fixed; the day pattern is a preference the generator satisfies where availability allows.
Why are some lessons still clustered after regeneration?
Blocks, part-time days, room limits, or stronger rules may leave clustering as the best valid outcome. Check the teacher's and room's available days first.
How is spread different from a same-day ban?
Spread pushes lessons onto more days across the whole week. A same-day rule only stops two lessons of one subject on a single day. You can use both.