Schedules are meant to be dynamic: they should have the flexibility to adjust to the changing project scenarios. Schedule logic is the driver that determines how dynamic and flexible a schedule is. Let’s explore an assessment to confirm your schedule is not ‘missing logic’.
A helpful measure of the effectiveness of project schedules is the Defense Contract Management Agency’s (DCMA) 14-point assessment. This assessment was developed to examine a schedule’s quality and/or soundness. Schedules that are judged high quality, as determined by the 14-point assessment guidelines, have a significantly higher probability of success. The 14-point assessment seal of approval ensures a well-built, and, therefore, practical schedule.
The first criteria and a major metric in the 14-point assessment looks for ‘missing logic’. Schedule logic defines the relationships between activities. The goal is not to mandate a start and finish date for each activity, but to implement logic that allows scheduling software to calculate each activities start and finish dates. ‘Missing logic’ results in a static schedule that does not react well to changing project situations.
This article investigates the ‘missing logic’ DCMA assessment as a measure of sound project scheduling.
Schedule logic, i.e. the relationships between tasks, describe the cause-and-effect dependencies between activities. A dependency in fact is the relationship between the (start or) finish of one task and the start (or finish) of another task. Again, the dependency reflects the cause-and-effect or logical relationship between two tasks. This logical relationship is detailed using the precedence diagram, Figure 1.
In this diagram task A is the independent cause or driver and task B is the dependent effect or driven. There are four task relationships described by the precedence diagram: start-to-start, finish-to-start, start-to-finish, and finish-to-finish. Precedence diagrams of each are displayed in Figures 2, 3, 4, and 5 for your reference.
The most common relationship between tasks is the finish-to-start dependency, Figure 3. Here the independent task, e.g. write report, drives (or causes) the start of the dependent task, e.g. print report. Refer to the following blog for a more in-depth discussion on task dependencies The Cause and Effect Task Dependency Paradigm.
A common mistake is to think of dependencies as the chronological sequencing of activities. This is why we emphasize that dependencies describe the cause-and-effect relationship between activities. Think cause-and-effect or independent driver and dependent driven. This enables you to understand start-to-start and finish-to-finish relationships that appear to occur simultaneously. In both these relationship types the predecessor is the defined cause or driver and the successor is the resulting effect or driven.
Many times project managers have a particular date in mind when they schedule certain tasks, e.g. a date to mobilize equipment and crew. This leads them down the faulty path of mandating a particular start date for their activity using a constraint. You do not want to do this. Constraints add additional stipulations on tasks that make the schedule static and not dynamic. Insertion of numerous task constraints into a schedule is like putting a straight-jacket on the schedule. The schedule becomes inflexible and immobile or unable to adjust or react well to inevitable schedule changes.
Good flexible schedules have task start and finish dates based upon task estimated durations and the dependencies between tasks. This way when a task slips or extends the scheduling software is able to recalculate the start and finish dates of all tasks in the entire schedule. Good scheduler’s therefore rely on scheduling software, along with realistic estimated task durations and sound dependencies, to compute task start and finish dates.
Another negative of inserting numerous task start and/or finish constraints in your schedule is that you lose your critical path. It is important to know your schedule’s critical and longest path in support of schedule optimization efforts. You optimize your schedule by shortening durations or adjusting relationships of tasks along the longest path through the schedule. A schedule that has constraints instead of dependencies will be disjointed and not have a continuous longest path through the schedule. Not good!
This brings us to our topic of discussion; the DCMA 14-point assessment and ‘missing logic’. The ‘missing logic’ criteria shows how well (or poorly) schedule tasks are linked together. The way to confirm a schedule is dynamic (or well linked and flexible) is to verify that in general every activity has both a predecessor and a successor (so no apparent dangling activities). Every rule has exceptions and in this case the caveats are the program start (has no predecessor) and program completion (has no successor). Also, although constraints are discouraged, a contractually defined constraint, e.g. contract completion date constraint, is acceptable.
The goal of the ‘logic missing’ assessment is to check all tasks for a possible missing predecessor, successor, or both. The assessment is adamant; the test fails if any activity (other than the start and finish of the schedule) doesn’t have both a predecessor and successor. In other words, all tasks must be logically connected to pass. And task start and finish dates, therefore, should proceed from the schedule network logic. Task start and finish dates (with minor exceptions) must not be limited by arbitrary constraint dates.
A scheduler’s major duty includes insertion of task dependencies, so task dates are driven by logic and not, again, arbitrary constraints. You want schedule logic to hold task dates in place, and not constraints.
The ‘missing logic’ assessment simply looks for tasks having no predecessor, successor, or both. Exceptions aside, this inspection ensures a logically driven dynamic schedule, i.e. a flexible schedule that reacts well or promptly to schedule updates. It also discourages scheduler’s tempted to leave, e.g. ‘pre-warranty conference’, tasks dangling without a successor past the contract completion date. The ‘missing logic’ assessment mandates these types of apparent dangling tasks be logically tied into the completion of the project.