What takes longer: building a schedule or revising a schedule??
Most operations analysts believe revising a schedule takes more time.. But why? The answer is that things seldom go as planned. Feeding the actual circumstances back into the planning process necessitates replanning. This feedback loop is apparently ignored by some of the scheduling tool designers. When you think about it, the replaning process is like a control loop where the errors must be compensated and damped out in a stable manner. Fixing these “errors”, which are the difference between the planned vs the actual, ends up being the biggest job of the scheduler.
What are the differences between the requirements for planning and re-planning? The first difference is that planning occurs with an empty timeline, and replanning occurs with a partially populated timeline. Second, in the re-planning situation, the activities that must be scheduled are a subset of the original activities. They are only the ones affected by the fact that the actuals are different from the planned.
If we look at the scheduling tools that are out there, many can’t schedule into the holes in the timeline that are left when the affected activities are unscheduled. Simulation-based schedulers are like this. They require that all activities be removed from some point in time onward. They don’t schedule around the activities that didn’t need to be unscheduled. This limitation causes more upheaval from the re-planning process than is necessary. The revised schedule may look very different from the original.
Do scheduling tools help users identify which activities are affected by the difference between the planned vs the actual situation? Some do and some do not. The task of identifying which activities need to be rescheduled is a major effort. And because the logic needed to make this determination is a recursive process of chasing links of common resource usage and successor relationships, human schedulers seldom do this with precision, choosing often to just unscheduled everything downstream, – whether needed or not.
Here are the take away questions: How efficiently do you do rescheduling? And, how well do your tools help with that process? Ask yourself: Is your rescheduling efficient? And are you using the right tools? Let’s talk about it.