Theory of Constraints Handbook - James Cox Iii [68]
For example, if several projects are simultaneously in execution, different departments might prioritize their work differently. All projects can make some progress but then become stuck at integration points where work-streams from different departments have to come together. Task priorities within departments could also get unsynchronized, in which case even the departmental work-streams would take longer. Unsynchronized priorities also create schedule conflicts, which can cause the individual resources to multitask, which results in lower quality.
If fewer projects are in execution, the chances are much higher that task priorities within and across departments are synchronized. The higher the WIP, the smaller the chances that task priorities will be synchronized!
Therefore, the first rule for execution success is: Limit the number of projects being run at a time. Projects should be staggered based on the most limiting resources because at any time only as many projects can be executed as you can get through those constraints. Any extra projects will only spread resources more thinly and destroy synchronization. Enforce this rule even if it means leaving some resources idle!
Rule 2 Buffering: Discard Local Schedules and Measurements, and Use Aggregate Buffers
The traditional project management approach is to turn task schedules and estimates into commitments. It assumes that if people are held accountable, they will finish individual tasks on time and on budget, and the entire project will consequently be on time and on budget.
Unfortunately, this traditional approach only leads to longer projects while causing execution to become more unsynchronized:
In planning, accountability for task-times causes people to include contingencies in their commitments—they have to plan for uncertainties as well as the reality that most of this time will be spent waiting for one thing or another. That is how project plans are extended.
In execution, resources now not only are scattered across too many projects, but also have an incentive to work on easy tasks—tasks that will help them beat or meet their estimates—instead of working on tasks that are most critical to the project.
Therefore, the second rule for execution success comes down to: Allow individual tasks to exceed their planning estimates. To protect projects from task delays, buffers are inserted before integration points and at the end of the project. With lower WIP, with the pressure to meet estimates gone, and with buffers to take care of uncertainties, the contingencies embedded inside task estimates are no longer needed and can be stripped out.
Not only does this second rule allow for shorter project plans (because buffers are smaller than the sum of task-level contingencies), execution becomes easier as well. With shorter project plans there is significantly less pressure to start projects as soon as possible; extra time can be used to get ready for execution through better preparation.
Rule 3 Buffer Management: Use Buffers to Measure Execution, and Drive Execution Priorities and Managerial Interventions
With low WIP and adequately buffered project plans, a single priority system can be firmly established in execution. The essence of the third rule is simple, but profound: Prioritize tasks according to buffer consumption. The highest priority is given to project legs that are consuming buffers at the fastest rate.6 When every person and department follows these priorities, they are all synchronized—automatically!
Buffer-based priorities not only are synchronized, but they also cause project status to be reliable. If resources work on the right tasks at the right time, it is assured that current project status is an accurate predictor of the future—despite uncertainties, most of which can be absorbed into the properly sized buffers. If recovery actions are initiated whenever buffers are