By definition, a schedule is laid out through time because of constraints.
Most constraints are time-based. Things take time to do and as we all know, time moves on no matter what. Time imposes constraints on the duration work requires, on the moment that work should be done, and finally on the person that will do the work since that person needs to be there and be available in order for the work to be done.
There are constraints related to the sequence of the work to be done. Sometimes, things need to be done in a certain order. I mentioned earlier that you can change a plan at any time until it’s actually happened. Well of course, there are different obstacles to changing a plan at the last minute. You can’t open the door on the highway and say « OK, I’m getting off » if you’re still at full speed. You’ll have to announce yourself, so that the steps for you to get off can be taken in the right order (like pulling over…).
The most widely known constraints are the ones concerning employees. An employee can only work so many hours in a day or in a week; an employee needs to rest; needs breaks; needs vacation; etc. These constraints are usually explained in an employee handbook or in a union agreement. The problem usually lies in the interpretation of these constraints as they are written with the perspective of human resources. They are not formulated for a planner in charge of applying them.
A strategic constraint negotiated at the company level may be difficult to apply at the schedule level. For example, if the company adopts a car pooling policy to reduce tardiness on the job, that’s great at an executive level and seems like it works for all employees. But if you have a department where employees share different shift start times, this policy imposes additional constraints to the planner. That planner now needs to schedule the same shifts to people who live close together so they can take advantage of car pooling as the policy now says. You can imagine the additional headaches for that planner to accommodate everyone while making sure the business operates efficiently. Some of my future entries will detail the constraints planners have to deal with and how to best express them.