What are the types of logical relationship?

Four logical relationships are possible. These relationships can be remembered if you use the same statement to describe the relationship and simply substitute the letters designating the relationship. The statement is: The independent activity must (first letter of the relationship) before the dependent activity can (second letter of the relationship), as shown in Figure 2-4.

Finish-Start Relationship (FS)

Most projects that you are likely to encounter will use the logical relationship of finishstart more often than any other relationship. This relationship is stated like this: The independent activity in the relationship must finish before the dependent activity can start.

This simply says that where there are two activities connected by an arrow, the one that is connected to the tail of the arrow must be finished before the dependent activity connected to the head of the arrow is allowed to start. It does not say that the dependent activity must start at that point; it could start later, but it is not allowed to start any sooner than the finish of the independent activity.

For example, I have two activities to complete in my project, which is to construct a wedding cake. The activities are to make the cake and put on the frosting. The finishstart relationship says that I cannot start putting the frosting on the cake until I have baked the cake layers. Notice that I could, logically, put the frosting on any time after that. The relationship constricts the start of the activity of frosting the cake to be no sooner than the finish of baking the cake.

Start-Start Relationship (SS)

The start-start relationship is stated in the same way as the finish-start relationship except that the word start is substituted for finish. The relationship is stated like this: The independent activity in the relationship must start before the dependent activity can start.

This simply says that where there are two activities connected by an arrow, the one that is connected to the tail of the arrow must start before the activity connected to the head of the arrow is allowed to start. It does not say that the dependent activity must start then. The activity could start later than that time, but it is not allowed to start any sooner than the start of the independent activity.

Figure 2-3. Precedence diagram.

Mon 7/10/00

Thu 7/13/00

Mon 7/10/00

Thu 7/13/00

Figure 2-4. Precedence relationships.

A must finish before B starts + 5 days

C must start before D can start

E must finish before F can finish

G must start before H can finish

E must finish before F can finish

G must start before H can finish

Leads or lags add or subtract time to the arrowhead event.

For example, using the above example of constructing a wedding cake, I do not want to apply the frosting to the cake until the master chef is on scene. The two activities then are: (1) apply frosting to cake, and (2) master chef supervises cake construction. The start-start relationship says that I cannot start putting the frosting on the cake until I have the master chef present. Notice that I could, logically, put the frosting on any time after that. The relationship constricts the start of the activity of frosting the cake to be no sooner than the beginning of the master chef supervising the cake construction.

Finish-Finish Relationship (FF)

The finish-finish relationship is expressed in the same way as the finish-start relationship except that the word finish is substituted for start. The relationship is stated like this: The independent activity in the relationship must finish before the dependent activity can finish.

This says that where there are two activities connected by an arrow, the one that is connected to the tail of the arrow must finish before the activity connected to the head of the arrow is allowed to finish. It does not say that the dependent activity must finish then. The activity could finish later than that time, but it is not allowed to finish any sooner than the finish of the independent activity.

For example, using the example of applying the frosting to the wedding cake, I must have the master chef there until the frosting is complete so that he or she can approve it.

The master chef is then restricted from finishing the supervising activity until the frosting activity is finished. The two activities are: (1) apply frosting to cake, and (2) master chef supervises cake construction. The finish-finish relationship says that the master chef cannot finish supervising the cake construction until the frosting is completed. Notice that I could, logically, have the master chef continue supervising after that. The relationship constricts the finish of the master chef supervising activity to be no sooner than the finish of the frosting activity.

Start-Finish Relationship (SF)

The start-finish relationship is very seldom used and has even been dropped from some of the project management scheduling software packages. This relationship is stated in the same sentence as the finish-start relationship except that the words start and finish are substituted for finish and start. The relationship is stated like this: The independent activity in the relationship must start before the dependent activity can finish.

This says that where there are two activities connected by an arrow, the one that is connected to the tail of the arrow must start before the activity connected to the head of the arrow is allowed to finish. It does not say that the dependent activity must finish then. The dependent activity could finish later than that time, but it is not allowed to finish any sooner than the start of the independent activity.

For an example, using the wedding cake example again, the project is still to construct a wedding cake, and the activity is to apply the frosting to the cake. We do not want to finish applying the frosting to the cake until the master chef is on the scene. The two activities then are: (1) apply frosting to cake and (2) master chef supervises cake construction. Remember that the start-start relationship says that I cannot start putting the frosting on the cake until I have the master chef present. The start-finish relationship says that I can start putting on the frosting of the cake before the master chef is present, but I am not allowed to finish putting on the frosting until the master chef has started supervising. Notice that I could, logically, start putting the frosting on any time before the master chef begins to supervise. The relationship constricts the finish of the activity of frosting the cake to be no sooner than the beginning of the master chef supervising the cake construction.

These relationships must be available to project managers and schedulers in order to be able to schedule all of the real relationships that are necessary to properly schedule a project. They are seldom used until attempts are made to reduce total schedule time.

In the examples involving frosting the cake, I related the frosting of the cake to the presence of the master chef to supervise the operations. At first the relationship was a start-start relationship, in which the frosting operation had to wait until the master chef began supervising. If we were trying to shorten the schedule, one of the things that might help would be to change the relationship between these two activities to a start-finish relationship. This would allow the frosting of the cake to begin much sooner but still require that the master chef supervise the completion of the activity.

Leads and Lags

To complete our discussion of relationships between schedule activities we must discuss leads and lags. Leads and lags are delays that are imposed in the relationship between the independent and dependent activity. They can help to shorten schedules as well as allow for delays between activities. Leads and lags are designated by adding a plus for lags, and a minus for leads, as well as the number of time periods that the lead or lag adds to the schedule.

A lag causes the dependent activity of the pair of activities in the relationship to have a designated number of time periods added to the start or finish of the dependent activity. A lead causes the dependent activity of the pair of activities in the relationship to have a designated number of time periods subtracted from the start or finish of the dependent activity.

For example, in the two activities previously discussed, baking the cake and putting the frosting on the cake, we established a finish-start relationship between the two activities. This said that we could not apply the frosting until the cake was baked. This is all right if the baking the cake activity included the time for the cake to cool. If it did not, and the cake activity ended when it was removed from the oven, then it would be necessary to insert a lag between the two activities. It is not possible to put the frosting on a hot cake, since it would melt and make a mess. This may be necessary because the cake baker would like to have closure on the baking activity and go about doing other things, and we may not want him to be responsible for waiting until the cake cools.

We would change the relationship from an FS to an FS + 1. This would force the schedule to allow one time period between completion of the baking activity and the start of the frosting activity.

A lead, on the other hand, allows the dependent activity to start sooner than the logical relationship would normally allow. In the example showing the start-finish relationship, we wanted to show that the frosting activity could start sooner in this relationship than if it used a start-start relationship. The problem with the start-finish relationship is that the frosting activity could start very much earlier than the supervising activity. The result of this might mean that the person responsible for the frosting cannot get closure on the activity until the master chef arrives. Another way to show this relationship is to make it an SS — 1 relationship. This means that the frosting operation could start as early as one time period before the master chef arrives.

Continue reading here: Diagramming Relationships

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