The very definition of a project includes a set of inter-related activities done by a group of people (according to the PMI®). Activities are linked to each other using relationships in the sequence in which those activities must be performed. There may be one or more paths of related activities, but one subset of them will be critical, meaning that the rest of the schedule is driven by the activities on that path.
There are four ways to relate activities to one another (types of relationships):
- Finish to Start
- Start to Start
- Finish to Finish
- Start to Finish
Relationships will be described using three abstract activities, called A, B, and C.
This is the most logical and most often used relationship type. In this case, A must finish before B starts. This is an expression of a relationship between these two activities only. For example, the frame of a building must be put up before any electrical work can begin; or the rig must be assembled before drilling can begin; or a specification must be written before writing code can begin.
This is the relationship when two tasks must begin at the same time. For example, as soon work begins on a job site, a nightly watch must be set. Another example is when procurement and resource acquisition may both begin at the same time. Another way of saying this is: as far as A is concerned, the start of A controls the start of B. Many times these are found following a milestone in a project schedule.
This is when two tasks must finish at the same time. One example is two lines of a railroad starting on each coast, with a planned meeting point at some predefined spot and time. In this case, your plan is that both of the lines be completed at the same time. By scheduling this way, you ensure that both crews arrive at the junction point simultaneously, and neither crew has to site idle waiting for the other. Another example is obtaining a permit and performing landwork, both of which must be complete prior to drilling. Another way of expressing this is: as far as A is concerned, the finish of A controls the finish of B. Many times these are found prior to a milestone in a project schedule.
This is the least common of the relationships, and can be a bit confusing. It means that B may not finish until A starts. A good way to consider this relationship is just in time.
As an example, imagine that you are building a facility in an area that has no power transmission lines. Activity B can be: run a generator to power up site X. Activity A can be: turning on a power line to the site, which is being built while the rest of the project progresses. B must continue as long as A has not started. It is not necessary that B finish once A starts, but it is allowed to finish once A starts.
Another example is when concrete the foundation cannot finish until plumb the foundation starts—so pipes can come up through the foundation for water lines.