Project Management Task dependencies
What are Project Management Task dependencies?
Task dependencies (sometimes called logical relationships) are the interrelationships between tasks on a project. These dependencies fix the order in which activities are carried out.
For ease of explanation, the dependencies that exist between tasks will be described in terms of just two: the predecessor task (the one before) and the successor task (the one with which the predecessor has a relationship). They are typically shown in a GANNT Chart.
Four types of task dependencies
- Finish to Start: a predecessor task must be completed before a successor task can begin. This is the most common task dependency.
- Start to Start: a predecessor task must begin before the successor task can start, or, said another way, the successor task can start only after the predecessor task has started. Sometimes this dependency is further modified by stating a fixed time after task A starts before which task B must start.
- Finish to Finish: a predecessor task must finish before the successor task can finish.
- Start to Finish: a predecessor task must start before the successor task can finish.
What causes Project Management Task dependencies?
Dependencies are created for different reasons. The constraints may be:
- logical (or causal) (e.g. a report cannot be proofread until it is written)
- resource-based (e.g. if there is only one editor, four books cannot be edited simultaneously)
- discretionary (e.g. a Project Manager prefers to do task A before task B, but could just as easily do the reverse).
Another type of task relationship is the parent–child relationship. This relationship is not a dependency, though there may be dependencies between the various child tasks. It exists when a large task (the parent) is broken down into its various smaller (child) tasks.
Displaying task dependencies
The dependencies between tasks, as represented on a GANTT Chart, will show some activities following one another, others occurring in parallel, and others starting or finishing within a time range set by another activity. This means there are often gaps of time between tasks. Sometimes, these time gaps are set delays— for example, lag time, whereby a certain amount of time, say 2 days, has to elapse after Task A starts (say, allowing a first coat of paint to dry) before Task B (applying a second coat of paint) can start. Other time gaps, such as lead time and float, allow the Project Manager to exercise flexibility in determining when they will be scheduled.
Identifying the critical path
Once a project schedule is developed, showing all task dependencies, resources can be meaningfully appointed to particular tasks, and the critical path for the project can be identified, using tools such as Critical Path Method, PERT or the Critical Chain Method.
To see an explanation of task dependencies and lag time, view: