Dependency (project management)
In a project network, a dependency is a link amongst a project's terminal elementscitation needed
The A Guide to the Project Management Body of Knowledge PMBOK Guide does not define the term dependency, but refers for this term to a logical relationship, which in turn is defined as dependency between two activities, or between an activity and a milestone1
- 1 Standard types of dependencies
- 11 Leads and Lags
- 2 Example
- 3 Advanced cases of activities dependencies
- 31 Maximal-Type Relationships
- 4 See also
- 5 External links
- 6 References
Standard types of dependenciesedit
There are four standard types of dependencies:
- Finish to start FS
- A FS B means "B can't start before A is finished", or in other words, "activity A must be completed before activity B can begin"2
- Foundations dug FS Concrete poured
- Finish to finish FF
- A FF B means "B can't finish before A is finished" or in other words "activity A must be complete before activity B can finish"2
- Last chapter written FF Entire book written
- Start to start SS
- A SS B = B can't start before A starts or in other words Activity B can start after Activity A has started2
- 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" The Practice Standard for Scheduling recommends, that "Typically , each predecessor activity would finish prior to the start of its successor activity or activitiesknown as finish-to-start FS relationship Sometimes it is necessarily to overlap activities; an option may be selected to use start-to-start SS, finish-to-finish FF or start-to-finish SF relationshipsWhenever possible, the FS logical relationship should be used If other types of relationships are used, they shall be used sparingly and with full understanding of how the relationships have been implemented in the scheduling software being used Ideally, the sequence of all activities will be defined in such a way that the start of every activity has a logical relationship from a predecessor and the finish of every activity has a logical relationship to a successor"2
SF is rarely used, and should generally be avoided Microsoft recommends to use SF dependency for just-in-time scheduling3 It can be easily shown however, that this would only work if resource levelling is not used, because resource levelling can delay a successor activity an activity, which shall be finished just-in-time in such a way, that it will finish later than the start of its logical predecessor activity, thus not fulfilling the just-in-time requirement
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
Leads and Lagsedit
Dependencies can be modified by leads, and lags Both leads and lags can be applied to all 4 types of dependencies
PMBOK defines lag as "the amount of time whereby a successor activity will be delayed with respect to a predecessor activity"
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
In accordance to PMBOK a lead is "the amount of time whereby a successor activity can be advanced with respect to a predecessor activity For example, on a project to construct a new office building, the landscaping could be scheduled to start prior to the scheduled punch list completion This would be shown as a finish-to-start with two-week lead"1
If you are building a building, you can't paint the walls before putting the water pipes into the walls
It would be much faster and less expensive, to put the pipes first, put the cement to actually build the wall around the pipes, and finally paint the walls
Advanced cases of activities dependenciesedit
Activity A and Activity B are said to have a Maximal-Type Relationship, if Activity B can start after Activity A, but with the delay of no more than X4 Real life examples, which are simulated by Maximal-Type Relation:
- Shoring of the trench has to be done not necessarily immediately after excavation, but within certain time, otherwise the trench will collapse
- Vaccination of baby has to be done not immediately after birth, but within certain time
- Renewal of the passport has to be done some time after the current one has been issued, but before it expires
- Invoice payment does not have to be done immediately, but within certain time after it has been issued
Maximal-type relationships are rarely implemented in the project management software, most probably because with this feature it is too easy to create contradictory dependencies
- Dependency structure matrix
- Outline of project management
- Project network
- Project planning
- Understanding Task Dependency Types
- ^ a b A Guide to the Project Management Body of Knowledge: PMBOK Guide Project Management Institute, Incorporated 1 January 2013 ISBN 978-1-935589-67-9
- ^ a b c d Practice Standard for Scheduling Project Management Institute 2011 ISBN 978-1-935589-24-2
- ^ "Microsoft article of SF links for Microsoft Project"
- ^ "ProJack Manager web site, describing maximal-type relationships"
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