Logical Relationship: A dependency between two project schedule activities, or between a project schedule activity and a schedule milestone. The four possible types of logical relationships are: Finish-to-Start, Finish-to-Finish, Start-to-Start, and Start-to-Finish. See also precedence relationship.
Precedence Relationship: The term used in the precedence diagramming method for a logical relationship. In current usage, however, precedence relationship, logical relationship, and dependency are widely used interchangeably, regardless of the diagramming method used. See also logical relationship.
Task relationships determine the start and finish dates of a task as it relates to other activities. The relations are labeled as Finish-to-Start, Start-to-Start, Finish-to-Finish and Start-to-Finish.
There seems to be relationships in every aspect of life. We have mathematical relationships, money and time relationships, food and wine relationships and even human relationships. So why not project management relationships.
Some relationships are simple while others are complex and baffling. Sometimes they are easier to figure out while others take a bit more noodling. These statements pertain to all relationships and even more so in project management.
Understanding these relationships and their use are critical to the project manager’s success. Why? Because most project managers don’t know they exist, don’t plan properly for their existence, and yet, the relationships exist and happen whether we like it or not.
Once we understand them, know how to apply them and plan according to their natural occurrence, the project schedule more closely reflects the reality of the project. The more closely we model future reality in our plans, the more likely we’ll succeed in our project and meet the stakeholders’ expectations. Additionally, understanding those helps us adjust our efforts and even bend reality at times to bring the project in line with what we want.
Understanding Alphabet Soup: FS, SS, FF, SF
The four relationships are:
- Finish-to-Start (FS)
- Start-to-Start (SS)
- Finish-to-Finish (FF)
- Start-to-Finish (SF)
Predecessor vs. Successor
Predecessor Activity: The schedule activity that determines when the logical successor activity can begin or end.
Successor Activity: The schedule activity that follows a predecessor activity, as determined by their logical relationship.
Before we can begin to understand the four relationships, we must understand predecessors and successors.
A Predecessor is typically the task that precedes other tasks. Successors typically occur after other tasks, or its predecessors. This is the common understanding of predecessors and successors.
In actuality, predecessors are the tasks that control the relationship between two activities. In fact, predecessors can actually occur after a successor. If that twists your mind a bit, stay with me as I explain how that can happen. I can guarantee you’ve been a victim of activity sequences where the predecessor occurred after its successor.
PMBOK® Definition: The logical relationship where initiation of work of the successor activity depends upon the completion of work of the predecessor activity. See also logical relationship.
Layman’s Definition: Once this task finishes, we can start the next one.
Practical Definition: In a Finish-to-Start relationship, the predecessor must finish before the successor can start. In fact, the predecessor’s finish date determines the Successor’s start date.
A Finish-to-Start relationship is typically displayed as follows:
FS is the relationship that occurs the most often within most project schedules. In fact, ninety-five percent (95%) of all tasks are related in a FS relationship. It is the most prevalent of the four relationships. As a result, many project management software packages such as MS Project use it as the default relationship. Unless specified otherwise, the software assumes activities relate in an FS manner.
It basically states once the predecessor finishes, the successor can start.
An example of FS relationships is car washing. We wash the car, dry the car and then wax the car. It makes no sense to dry the car before we wash it. And certainly waxing the car first only grinds the dirt into the paint rather than protecting it.
The activities in car washing naturally fall into a finish-to-start relationship. Of course, we could overlap the activities. In other words, we could start drying the car before we are finished washing it, but we do risk water overspray causing additional necessary drying. The overlapping of activities still follows the definition of finish-to-start relationship because the one spot being dried must be washed and rinsed first.
PMBOK® Definition: The logical relationship where initiation of the work of the successor schedule activity depends upon the initiation of the work of the predecessor schedule activity. See also logical relationship.
Layman’s Definition: We want these two tasks to start at the same time.
Practical Definition: Once the predecessor task starts, we can start the successor task.
A Start-to-Start relationship is typically displayed as follows:
In essence, they all sound the same. The start of the successor task is gated by the start of the predecessor activity. Until the predecessor starts, the successor cannot start.
But here is another meaning people miss. It simply states the successor cannot start UNTIL the predecessor starts. It does not mean it has to start at exactly the same time. It can occur sometime after the beginning of the predecessor’s activity.
An example of a start-to-start relationship might be tabulating results from some market research. In market research, we develop a survey, distribute the survey, and wait for responses to our survey. As we receive the responses, we enter the data into a database and tabulate the information. We do not need to wait for all the responses to return before tabulating the information.
In fact, in market research, a one percent response rate is considered good. Two to four percent response rate is super. But what it really means is we will not receive from 96 to 99% of the desired responses. If we used a finish-to-start relationship between the receiving responses and tabulating the results, we’d never tabulate the responses. Therefore, once the responses flow in, we can start tabulating the results and watch for early trends. This method is what they use during political elections trying to predict the winner before all the votes are counted. Programming this example into project management software would yield the following Gantt chart:
PMBOK® Definition: The logical relationship where completion of work of the successor activity cannot finish until the completion of work of the predecessor activity. See also logical relationship.
Layman’s Definition: We want these two tasks to finish at the same time.
Practical Definition: Once the predecessor task finishes, the successor task can finish.
A Finish-to-Finish relationship is typically displayed as follows:
When serving dinner, we typically experience, or want to experience, a finish-to-finish relationship. Usually, we want all the food to be ready for eating at the same time and placed on the table, arranged for the family to eat.
Using a finish-to-start relationship while preparing the food results in some portion of the meal cooked while other parts have not even been started. Using a start-to-start relationship while cooking the food can result in some items being over-cooked, under-cooked or just right. Therefore, the only relationship which works for having all the items cooked to the right doneness and placed on the table at the same time is a finish-to-finish relationship.
Let’s say our menu includes meat, potatoes and vegetables. Setting the oven at 350o, the meat takes 45 minutes, the potatoes 60 minutes and the veggies 15 minutes. To properly stage the cooking so all finish at the same time, we would put the potatoes in the oven first, wait 15 minutes, put the meat in, wait another 30 minutes and then add in the vegetables. Programming this example into project management software would yield the following Gantt chart:
PMBOK® Definition: The logical relationship where the completion of the successor schedule activity is dependent upon the initiation of the predecessor schedule activity. See also logical relationship.
Layman’s Definition: This task finishes when the next one starts, but not before then.
Practical Definition: Once the predecessor task starts, the successor task finishes.
A Start-to-Finish relationship is typically displayed as follows:
Read the definition again. The successor task finishes when the predecessor activity starts. Wait! How can the successor task finish, which means it started, before the predecessor activity starts? I don’t know about you, but this definition twists my brain sideways.
Fortunately, this relationship only occurs less than 1% of the time, so most people miss it, or simply don’t know it exists, but it does. I have seen many people trying to describe this relationship and frankly, most are wrong. So, I provide two here. Don’t go scurrying to the PMBOK for examples because it is conspicuously devoid of examples of these relationships except FS. Hmm…
Imagine you are a dinner boat cruise owner. The boat leaves the dock at 6:00 pm for the cruise around the city. As a smart owner, you open the ticket booth window for sales at 2:00 pm. Your goal is to sell out of all tickets before the boat leaves the dock.
At 5:00 pm, the captain of the boat calls you and says, “Hey boss, I’m stuck in traffic. I’m going to be late. Please let the customers know the boat won’t leave the dock until at least 7:30 pm.”
Six o’clock comes and all the tickets are not sold. As the owner, what do you do? Do you close the ticket sales window and wait for the boat to leave the dock? Do you close the window and go home? Or do you keep selling the tickets until the boat leaves the dock? Of course, you’d keep the window open selling tickets until the cruise is underway.
What determined the finish time of the ticket sales: the clock striking six or the boat leaving the dock? It is the boat leaving the dock. The clock reaching 6:00 pm is meaningless. The boat leaving the dock determines the relationship between the two tasks.
So we see from this example, the predecessor isn’t necessarily the task that comes first, but one that controls the task relationships. If we go back and review the other three relationships already described, we’ll see the same condition – the predecessor determines the relationship. In the start-to-finish relationship, the predecessor is the “main” event, the one determining the finish of the other activity. In the case of the ticket window opening, its opening is determined by the time of day or 2:00 pm. Whether the boat leaves on time or not does not impact the window’s opening.
As we saw, the window’s closing time was extended by the boat’s late departure. Programming this example into project management software would yield the following Gantt chart:
I understand most people reading this definition are not dinner boat cruise owners. So, a more practical example might seat the understanding into memory. Maybe not more practical, but one “closer to home.”
Remember back to the days you went to school. In order to test your knowledge of a subject, the teacher always gave a test. If you were like me, you’d wait until the last minute to cram for the exam. In fact, if the instructor showed up a bit late, you’d still be studying until the test paper hit your desk.
What determined the start of your study time? I never did figure that out, but in my case, it seemed to be panic. But I can tell you what determined the finish of my study time. The professor stating all study material must be put away. That, my fine readers, is a start-to-finish relationship.
Understanding the four relationships between activities in a project schedule helps model reality most accurately. Unfortunately, most project teams only use the finish-to-start relationship between tasks. This method does not accurately schedule the activities of a project.
When planning the steps in a project, actively discuss relationships between all tasks and accurately schedule them. Just as determining the duration of a task is important, relating it to the other things that need to be done is critically important for the success of the project. You might not use all four relationships within a project, but you should at least know they exist and consider them.
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