What Is a Dependency?

A dependency refers to the relationship between tasks or activities, where the completion of one task is reliant on the completion of another task. In other words, a dependency exists when a task can only be performed after another task has been finished. In project management, dependencies help determine the sequence and order in which tasks should be executed to ensure smooth project progression and successful outcomes.

With this in mind, it’s good practice to map dependencies on a dependency diagram to avoid points where teams are waiting around because timings don’t align. This helps you maximize efficiency. 

Dependency diagrams map four types of dependencies:

• Finish to start: You can begin Task B when Task A is complete.

• Finish to finish: You can finish Task B when Task A is complete.

• Start to start: Task B can start after Task A starts.

• Start to finish: Task A can complete after Task B begins.

Categories of dependencies in project management

The concept of dependencies comes in many forms or variations. These include logical, resource, preferential, and external dependencies.

Logical dependencies

A logical or causal dependency indicates that certain project steps must be completed in a specific sequence to achieve the desired outcome. An example of a logical dependency is the requirement to finish the design phase before starting the manufacturing phase of a product. 

Resource dependencies

A resource occurs when a task or activity depends on the availability of specific resources, such as personnel, equipment, materials, or funding, to be executed. In other words, the start or completion of one task is contingent upon the availability or completion of another task's required resources.

Preferential dependencies

Preferential dependencies are intentionally created or established based on preferences or strategic considerations rather than strict logical or resource constraints. Unlike other types of dependencies, preferential dependencies are subjective and based on the project team's judgment or decision-making. Preferential dependencies allow project teams to optimize project plans, resource allocation, or stakeholder priorities based on their expertise, experience, or specific project requirements.

External dependencies

An external dependency arises from factors outside the immediate control of the project team or organization. It denotes a reliance on external entities, events, or circumstances that can impact the project's progress or outcomes. This could be a regulation or legal contract you have to follow or something like having to stop due to industry strikes. External dependencies are out of your control.

Related terms

• Procurement

• Project risk

• Gantt chart

• Milestone

• PERT chart

Learn more with Google

If you’re looking to learn more about project dependencies and how you can plan for them, the Google Project Management Professional Certificate on Coursera is a great place to start. Learn the fundamentals of project management from industry experts at Google at your own pace, and get job ready in as little as six months.