Dependency (project management)
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In a project network, a dependency is a link amongst a project's terminal elementss.
There are four kinds of dependencies with respect to ordering terminal elements (in order of decreasing frequency of use):
- Finish to start (FS)
- A FS B = B can't start before A is finished
- (Foundations dug) FS (Concrete poured)
- Finish to finish (FF)
- A FF B = B can't finish before A is finished
- (Last chapter written) FF (Entire book written)
- Start to start (SS).
- A SS B = B can't start before A starts
- (Project work started) SS (Project management activities started)
- Start to finish (SF)
- A SF B = B can't finish before A starts
- (New shift started) SF (Previous shift finished)
Finish-to-start is considered a "natural dependency" whereas all the others are constraints imposed by the scheduler to reflect resource constraints or preferential dependencies. SF is rarely used, and should generally be avoided.
There are three kinds of dependencies with respect to the reason for the existence of dependency:
- Causal (logical)
- It is impossible to edit a text before it is written
- It is illogical to pour concrete before you dig the foundations of a building
- Resource constraints
- It is logically possible to paint four walls in a room simultaneously but there is only one painter
- Discretionary (preferential)
- I want to paint the living room before painting the dining room, although I could do it the other way round, too
Early critical path-derived schedules often reflected only on causal (logical) or discretionary (preferential) dependencies because the assumption was that resources would be available or could be made available. Since at least the mid-1980s, competent project managers and schedulers have recognized that schedules must be based on resource availability. The critical chain method necessitates taking into account resource constraint-derived dependencies as well.
In addition, these dependencies can be modified by leads, and lags. For example: When building two walls from a novel design, one might start the second wall 2 days after the first so that the second team can learn from the first. This is an example of a lag in a Start-Start relationship.
It may also be useful to specify lead time when tasks are performed in parallel in a Finish-Finish relationship. For example: The work for 'Document A' should finish 5 days before the work for 'Document B' so that the reviewers have time to read each individually. Although Document A and Document B may take different times to write, they will be planned to finish 5 days apart.