Fast Tracking and Crashing the Schedule
Once the scheduler has defined unique activities with realistic durations, the next step of defining activity relationships may have a big impact on the length of the project. The scheduler uses the precedence diagramming method to show the relationships or corresponding dependency between activities. Experienced schedulers thoroughly understand the relationships between activities and are skilled at applying tools for compressing the schedule. Schedule compression is a major component in creating a schedule that is both realistic and as short as possible.
A resource conflict occurs when two tasks can be performed at the same time, but they are scheduled as a predecessor and successor simply because there are not enough resources to perform both tasks at the same time. This is known as a resource constraint. A situation where one activity must proceed another is often referred to as a hard logic relationship or a mandatory dependency. A soft logic relationship are dependencies that are added by people that prefer to execute activities in certain order. This is because these dependencies are not mandatory and are defined by people. Soft logic relationships are also referred to as a discretionary dependency or preferential logic.
Dependencies can also be classified as external or internal. For example, an external dependency means that there is a dependency with an activity on another project or an external event.
Precedence Diagramming – Relationships
The precedence diagramming method is implemented to show activity dependencies. In a logic view, nodes or rectangular bars represent activities, and arrows show activity dependencies. These can be displayed other ways including on a Gantt chart. There are four types of relationships the scheduler can use to define the dependencies between activities.
- Finish-to-Start (FS): This is the most common type of relationship where one activity cannot start until another associated activity has finished. An obvious example is that installation cannot start until demolition is complete.
- Start-to-Start (SS): This is a relationship where one activity cannot start until another activity has begun. An example is that demolition cannot proceed until the safety plan has commenced.
- Finish-to-Finish (FF): In this relationship one activity cannot finish until another related activity has finished. A possible example from the research industry is that documentation cannot complete until experimental testing described by that documentation has finished.
- Start-to-Finish (SF): In this not so common relationship one activity cannot finish until another activity has begun. An example is the use of a generator that cannot cease until the power is activated.
Lag and Leads
Lags and leads have a significant impact on the length of the schedule. A lag describes a required time between activities. In the FS relationship this means that the second activity cannot commence until a scheduled period of time has elapsed after the completion of the first activity. The most common example is concrete, in which a lag is inserted between activities to allow the concrete time to cure before the successor activity can proceed. After the lag time period has expired then the second activity of installing framing on the concrete flooring can commence. A lead has the opposite effect on the schedule. A lead is the amount of time that an activity can start before the completion of the first activity. Many schedulers refer to lead as a negative lag. It is worth noting that leads are not typically recommended and are often avoided by experienced schedulers.
Many times a scheduler will organize activities in a series format where activities are performed one after the other with pure FS relationships. However, the scheduler should consider whether these activities could be performed in parallel. Fast Tracking is used when the scheduler decides to take risks and schedule activities in parallel, even though it is better to do them sequentially. A simple example would be to start construction before design is finished. These risks are taken to keep the project on track or to have a chance to finish earlier.
What do you do when fast tracking is not an option? How do you compress the schedule in these situations? Well, if the project does not have strict cost constraints the scheduler could add additional resources to certain activities to complete them faster. Extra crews are added to a task to increase daily output and, therefore, complete the task faster without changing the required serial relationship between project tasks. This invariably results in increased costs. Other examples include applying different technologies to accelerate critical tasks, or changing work calendars (to make Sunday a working day or have teams work additional hours).
Crashing is often applied to critical activities and it also makes sense to look at activity drags. Crashing almost always increases cost. In the end money, is traded for reduced time.
The ability or flexibility the scheduler has to adjust tasks to reduce time is partly dependent on the type of relationships between the tasks. There is no flexibility for hard logic relationships, but soft logic relationships can be adjusted.
Dependencies between tasks are defined by four relationships. Understanding and properly applying these relationships is important for accurately determining the length of the project.
Two tools at the scheduler’s disposal for compressing the schedule are fast tracking and crashing. Fast tracking requires a thorough review of activity precedence modeling to determine whether tasks in series can be performed in parallel. When other methods of compressing a schedule are not suitable crashing a scheduling will reduce time. But the project will incur increased cost for the time-savings.