Understanding Project and Task Dependencies

Project Managers reference project dependencies as the relationships between individual tasks in a project diagram. Preceding tasks must be completed before moving on to the next or succeeding tasks. All of the tasks required to achieve the project's scope are sequenced according to their dependencies for each other, and then resources are linked to the tasks and a project schedule constructed.

The project or task dependencies are critical for the following:

• Sequencing the work packages (tasks) in a project plan.
• Calculating the critical path (longest path/duration) of tasks in a project plan. 
• Identifying resource and scheduling issues and making supporting decisions. 
• Monitoring and managing as part of the overall project plan. 
• Identifying opportunities to accelerate the schedule via fast-tracking or crashing. 

Examples of Project Task Dependencies

While the analysis of task dependencies for a large project is a complex task often requiring computer software, consider a few simple examples to illustrate the concept. 

Example 1: Making Coffee

Imagine getting out of bed in the morning, yawning, and making your way to the kitchen to discover that you forgot to preset your coffeemaker to brew automatically. It is now up to you to shake off the sleep and start the coffee brewing. You know that you need to complete the following tasks:

• Add water
• Grind the coffee beans
• Measure the coffee into the filter
• Add a coffee filter
• Place the carafe on the heating tray
• Press brew

Of course, there is a proper order to this process. You would not press brew before completing all of the other steps. A proper sequence of events taking into account dependencies might be as follows:

1. Grind the coffee beans
2. Add a coffee filter 
3. Measure the coffee into the filter
4. Add water
5. Place the carafe on the heating tray
6. Press brew.

In this sequence of tasks, you could not measure the coffee into the filter before you ground the beans. Grinding the coffee beans is a predecessor activity for measuring the coffee into the filter. The measuring step is a succeeding activity. All of the tasks are predecessor activities to the step: Press brew.

Example 2: Installing a Lawn Sprinkling System in a Brand New House

Important tasks for this include:

• Install the sod (lawn).
• Dig the trenches for and install the sprinkling system hoses.
• Locate the sprinkler heads.
• Grade the lot.
• Fill the trenches.

The proper sequence of events taking into account task dependencies is as follows:

1. Grade the lot.
2. Locate the sprinkler heads.
3. Dig the trenches for and install the sprinkling system hoses. 
4. Fill the trenches
5. Install the lawn. 

Understanding the dependencies in this example allows the landscaper to plan for grading and digging equipment, ensuring the availability of supplies and scheduling the proper resource for each step.

Types of Project Planning Dependencies

There are four types of project planning dependencies. They establish relationships among the tasks. Below, they are listed in the order most often used"

1. Finish-to-start (FS): The first task must complete before the second task can start. For example, the task "Write code module 1" must finish before the task "test code module 1" can begin.
2. Finish-to-finish (FF): The second task cannot finish before the first task finished. The task "all code tested" cannot finish before the task "test code module x" finishes.
3. Start-to-start (SS): The second task doesn't start until the first task starts. The task "write training manual" must start before the task "write chapter 1 of the training manual" can start.
4. Start-to-finish (SF): The first task must start before the second task can finish. The task "assign coder for module 3" must start before the task "all work assigned" can finish.

Categories of Project Planning Dependencies

There are also dependencies with other projects. Planning dependencies fall into three categories: logical, resource-based, or preference. Below are three types of project planning dependencies:

1. Logical planning dependencies: These are dependencies that are logic-driven. You can't paint a wall before it's built. You can't test a code module until after it's written.
2. Resource-based planning dependencies
   These are dependencies where the task could be accomplished faster or sooner if you had more resources. The manual could be written faster if you had more than one writer. You could write more code except all the other developers are committed to other projects.
3. Preference planning dependencies
   These are tasks that could be scheduled differently, but the project manager chooses to schedule this particular order.

The Bottom Line

Project managers must be able to plan for and manage the dependencies among tasks in their projects. Dependencies impact scheduling and resource activities, and it is critical to understand these relationships when looking for opportunities to improve the project schedule.

Updated by Art Petty