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Dependencies and types of dependencies

Dependencies and types of dependencies

Owned by Mariia Korolishyna
Last updated: Jan 30, 2025
2 min read

What is dependency?

Dependency is a link between two project tasks. In other words, a dependency is a logical relationship between two activities, or between an activity and a milestone.

 

Standard types of dependencies

There are four standard types of dependencies:

  1. Finish to Start (FS)

  • A FS B means "activity A must finish before activity B can begin" (or "B can't start until A has finished").

Dependency-FS.png

 

  1. Finish to finish (FF)

  • A FF B means "activity A must finish before activity B can finish" (or "B can't finish before A is finished") .

Dependency-FF.png

 

  1. Start to start (SS).

  • A SS B means "activity A must start before activity B can start" (or "B can't start until A has started").

Dependency-SS.png

 

  1. Start to finish (SF).

  • A SF B means "activity A must start before activity B finishes" (or "B can't finish until A has started")

Dependency-SF.png

 

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 activities)(known 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) relationships....Whenever 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".

 

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