Detailed planning – Dependencies (4 basic types)
Finish to start
This is by far the most common.
Task ‘A’ must finish before Task ‘B’ begins.
For example:
The primer paint coat must be completed before the finish coat is applied.
Start to finish
This is rare.
It is often complicated to put into a schedule and is best substituted by Finish to Start with suitable lags.
Task ‘A’ must start before Task ‘B’ can finish.
For example:
This may apply if completion of task ‘B’ relies on materials or equipment that require a period of preparation (task A) and it is important that they are prepared just ahead of requirement.
Finish to finish
Quite common.
Task ‘A’ and Task ‘B’ finish at the same time (or strictly Task ‘A’ has to finish before Task ‘B’ can finish).
This is useful if you have two or more activities running in parallel that rely on some common element to finish at the same time.
For example:
Two reports cannot complete until they receive a common piece of information.
Start to start
Quite common.
Task ‘A’ and Task ‘B’ start at the same time (or Task ‘A’ has to start before Task ‘B’ can start).
For example:
Both tasks await the arrival of a common component before manufacturing can start or a decision point has been reached and tasks ‘A’ and ‘B’ cannot proceed until a decision has been made.
Task dependencies are tricky and need a lot of thought for the final schedule.
Tasks very commonly have dependencies on many tasks.
Often tasks do not fit quite neatly into the above brackets.
Task lags are often used, both positive and negative.
Some examples are given next as displayed in MS Project.