For practical reasons, most scheduling problems are an abstraction of the real problem being solved. For example, when you plan your day, you schedule the activities which are
; that is you schedule the activities which are essential to the success of your day. So you may plan what time to leave the house to get to work, when to have meetings, how you share your vehicle with your spouse and so on. On the other hand, you probably do not consider the activities that are easy to arrange like brushing your teeth, going to the shops, making photocopies and other such tasks that can usually be accomplished whenever you have the time available. Scheduling all of these activities at once is often too complicated. Instead, a simpler schedule is produced by considering only the critical activities. However, if a schedule goes wrong, it is often because an activity turned out to be critical but was not scheduled. We typically learn which activities are critical by experience and create an abstract scheduling problem which includes all known critical activities. Instead of scheduling the non-critical activities we estimate their effects in the abstract scheduling problem.