Instructions.
Last modified on 2 November 2021.
These are some small instructions which you have to follow in order to input a solvable data set.
If you get an impossible timetable and your institution allows beginning later for students, please reconsider your students (set) early constraints to allow more beginnings at second hour. Also, removing or weakening other constraints might help. If FET cannot find a good timetable for your school, make sure to report this, as FET needs to be improved. Maybe small changes to your datafile can bring good solutions, or maybe there are aspects which have to be changed in FET.
An impossible timetable might also be caused by incorrect years division. Please check statistics/students for all subgroups, each subgroup should have the necessary hours, not less. If you have for instance some subgroups with less than say 20 hours per week you might get an impossible timetable, probably because of incorrect division of years and incorrectly added activities. All the subgroups are independent and should have an amount of working hours per week close to the average for all subgroups. Please see FAQ for more information.

Please select correctly the days and hours (working periods). To ensure best performance, it is best to keep the number of hours (periods) per day to your required needs, not larger. It is a good practice to choose a value for the number of days and number of hours from the beginning (you can change their names anytime you want afterwards).

To say that component activities of a larger split activity are not in the same day, FET automatically adds a constraint min days between activities with default 95% weight. If you want to ensure no two activities of same type are in the same day, you will have to modify the weight from 95% to higher values (see these constraints in the Time constraints menu>Activities>Min days between a set of activities constraints. You have there a powerful filter to change many constraints with a few clicks).
To specify that some activities must be in different days, the min days between activities must be 1. To specify that some activities must be separated even more, you can increase the min days to 2 (or even more, but probably not needed). Min days = 2 means that activities will be at least 2 days apart from each other (so there is another day between them). It is recommended that when inputting activities which are split into only 2 subactivities per week, add min days = 2. Or you can, after introducing the activities, to modify the min days value in more constraints at once by using the powerful filter in constraints min days between activities dialog (select old number of activities 2 and new min days 2, maybe also raise weight percentage to 100%).
You can use for the constraint min days the weight you want. It can be 95%, 99.75% or even 100%. If you specify a value below 100% (even 99.75% or more) for an impossible constraint, FET will be able to detect that it is impossible and obtain a timetable in the end, so the weight is subjective. So, the best way would be to choose a 99.75% for all constraints, but the time of generation might be larger than with 95%. You could try at first the 95% minimum recommended value, then highten the weight percentage up to maybe 99.75% or 100%. I am not sure here, I have not enough sample files (please contribute with advice).
If you would like to change the default 95% for another value for some constraints of type min days, the easiest way is in Data/Time constraints/Min days between activities dialog, where starting with version 5.6.1 there is a very powerful dialog, where you can specify a filter (old values) and new values for the selected constraints.
You might want to choose different weights for different constraint min days (for instance, higher on subjects with less activities per week)
In versions 5.5.8 and higher, you can add 2 constraints min days for the same split activity. In add activity dialog, select min days 2 (or 3) and you get the chance to add another constraint min 1 days (or 2). For instance, if you have 3 activities per week from the same group, you can add 2 constraints, min 2 days and min 1 day, both with 95%. This will ensure that in 99.75% of cases the min 1 day will be respected.

If you have a course lesson with say 45 activities per week, difficult lessons (Maths), which you would like to schedule in the beginning of the day (say 3 out of 45 need to be in the first half of the day), the best approach is to use constraint subactivities preferred time slots or starting times, specifying possibly the subject and component number: 1, 2 and maybe 3, adding 3 constraints for this subject (see FAQ for more details). You can also add constraints for component numbers 3, 4 and 5 for subject Maths (this way, if activity is split into 4 it will get 2 early, if it is split into 5 it will get 3 early  a nice trick). Similarly, if you have say Bio with 1 or 2 splits per week, you can add constraint with component number = 2.

From the way the algorithm is conceived, the automatic generation is faster if you use students set (or teacher) not available and/or constraint activity(ies) preferred time slots or preferred starting times to specify impossible slots, in addition to possible use of students (set) or teacher(s) max hours daily, whenever it is possible. For instance, if you know that year 5 will only have hours from 8:00 to 13:00, it is better to add students set not available in addition to students set max hours daily. So, if possible, try to follow this advice. This trick helps guide FET better towards a solution.
Each constraint of type not available or preferred times which filters out impossible slots might bring an improvement in speed.

Constraint students (set) early: if you input only partial data, you might get impossible timetables (see FAQ for details).

For teacher(s) gaps: please note that FET will take care of teacher(s) gaps only if you tell it so, by adding constraint teacher(s) max gaps per week. It is normal to add such constraint(s) after you saw that the data is solvable, using for max gaps a value which seems acceptable, lowering it as you find timetables.

Rooms: You might firstly try to generate a timetable without rooms, to see if it is possible, then add rooms and rooms related constraints.

It is possible to work with institutions in which the students work in shifts (for instance, lowest forms in the morning and highest forms in the afternoon). Please see the Help/Frequently Asked Questions the solution to how to do that (the essence is to add corresponding constraints students set not available).

The new algorithm (after version 5.0.0) does not accept fortnightly activities. But you might apply a trick. Please see menu Help/Frequently Asked Questions menu (it has the description of a possible trick) (the essence is to make a weekly activity with the sum of teachers and students of both fortnightly activities)

Even if it takes a long time to generate your timetable with a set of constraints, strengthening the constraints might not slow the generation too much. You are adviced to try with stronger constraints after you obtain timetables, because you can obtain better timetables this way.

If a constraint is allowed values under 100%, you can use any weight, even fractional numbers like 99.75%. It might help in constraints like min days between activities, preferred rooms or max hours daily.

Each room can host a single activity at the same time. If you have a large room which can host more activities at the same time, you need to use a small trick. You can add more rooms representing this large room and add corresponding constraints preferred rooms. There is an entry in the FAQ about that.

If teachers are getting days with only one hour of work, you might want to add constraint teacher(s) min hours daily (probably with 2 hours), but please make sure your timetable is possible. This constraint is smart, it only considers nonempty days.

If you have for instance 7 hours of Maths on a 5 days week (more lessons than days), please respect the correct way to add these as a split activity. See question Q127March2008 from FAQ. It is important!

It is recommended to remove redundant min days between activities constraints after adding constraints same starting day or time (read Help/Important tips).

If you want for instance teachers (or students) not to have more than 2 times per week activities in the last hours, you have to use the constraint teacher(s) or students (set) hourly interval max days per week. Please read FAQ for more details.

If you have activities which you want to put in the last slots of a day (like say the meetings with the class master), please use the constraint a set of activities end students day (or singular activity ends students day).

If you use the not perfect constraints activity tag max hours daily or students max gaps per week (there are 4+2 types of constraints in this category), use them with caution not to obtain an impossible timetable. If the timetable is impossible, it may be because of them. You are advised to add such constraints only in the end, after you are sure that the other constraints are good and the timetable is possible.