In a project network, a dependency is a link amongst a project's terminal elements. 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 doesn't start before A is finished
- (Foundations dug) FS (Concrete poured)
- Finish to finish (FF)
- A FF B = B doesn't finish before A is finished
- (Last chapter written) FF (Entire book written)
- Start to start (SS).
- A SS B = B doesn't start before A starts
- (Project work started) SS (Project management activities started)
- Start to finish (SF)
- A SF B = B doesn't finish before A starts
- (New shift started) SF (Previous shift finished)
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
- 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
Traditional critical path-derived schedules are based only on causal (logical) dependencies. The critical chain method necessitates taking into account resource constraint-derived dependencies as well. Time related information can be used in conjunction with any of the relationships for any of the reasons outlined above. This is known as a lead or lag. For example: When building two walls from a novel material, one might start the second wall 2 days after the first so that the second team can learn from the first. It is also useful to use lead/lag when tasks are in parallel in a Finish-Finish relationship. For example: The document for 'Activity A' should finish 5 days before the document for 'Activity 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.
