Understanding how tasks relate to one another forms the basis of effective project management. A project dependency is the relationship where the initiation or completion of one task is constrained by the status of another specific task. These logical connections structure the project schedule, ensuring that activities are performed in a sensible order. Grasping these relationships makes it possible to build a realistic timeline and allocate resources efficiently.
Defining Project Dependencies
A dependency establishes a logical sequence of work, defining which tasks must precede others. The task that must be completed or started first is known as the predecessor, and the task that follows is the successor. This linkage ensures that tasks are not attempted before the necessary inputs or conditions from earlier activities are ready. For example, frosting a cake (the successor) is dependent on baking the cake (the predecessor) being finished.
Dependencies are the constraints that shape the project workflow. Without clearly defined dependencies, a project schedule is merely a list of tasks without an accurate path to completion. Documenting these links is fundamental to developing a reliable schedule that all stakeholders can follow.
The Four Core Dependency Types
Project scheduling relies on four standard types of logical relationships between a predecessor task (A) and a successor task (B). These relationships define exactly how the start or finish of one task affects the start or finish of the other. Identifying the correct type is necessary for creating an accurate project schedule network diagram.
Finish-to-Start (FS)
The Finish-to-Start dependency is the most common relationship, stating that Task B cannot begin until Task A is completely finished. This reflects a sequential flow where one activity must deliver a tangible output before the next can utilize it. A clear example is that a construction crew cannot start building the walls (Task B) until the foundation (Task A) has been laid and cured.
Start-to-Start (SS)
A Start-to-Start dependency means that Task B cannot start until Task A has started. The completion of Task A is not required, only the initiation of the work. This relationship is useful for activities that can be performed in parallel once the initial effort has begun. For instance, writing software code (Task B) might start as soon as the design documentation (Task A) begins.
Finish-to-Finish (FF)
The Finish-to-Finish dependency dictates that Task B cannot be completed until Task A is also finished. This link is frequently used when two activities are closely related and must conclude simultaneously to move to the next project phase. In a building project, the installation of electrical wiring (Task A) might need to be finished before the final inspection (Task B) can be finished.
Start-to-Finish (SF)
The Start-to-Finish dependency is the least common of the four types, meaning Task B cannot be finished until Task A has started. This relationship is typically used in transitional or handover scenarios where a support function must remain active until the replacement function begins. For example, an old IT system (Task B) cannot be fully decommissioned until the new IT system (Task A) is officially started and operational.
Understanding Dependency Relationships
Dependencies are classified by their source (internal or external) and flexibility (mandatory or discretionary). Internal dependencies are within the project team’s control, such as the handoff of a document.
External dependencies are imposed by outside parties, like vendors or regulatory bodies. These often involve waiting for a third-party deliverable and present higher risk because the project manager has limited control.
Mandatory dependencies (hard logic) are non-negotiable physical or contractual requirements, such as pouring the foundation before framing the walls. Discretionary dependencies (soft logic) are based on best practices and can be altered to optimize the schedule.
Why Dependencies Are Critical to Project Success
Properly identifying dependencies determines the project’s timeline and risk profile. Dependencies are the inputs for calculating the Critical Path, the longest sequence of activities establishing the earliest possible project completion date. Any delay to a task on the Critical Path will delay the entire project’s finish date.
Failure to accurately map these links leads to scheduling inaccuracies and missed deadlines. Understanding which tasks have float, or slack time, allows the project manager to prioritize effort and manage risk effectively and prevents wasted effort or rework.
Practical Steps for Identifying and Mapping Dependencies
Identifying dependencies begins with a detailed decomposition of the project scope. Using the Work Breakdown Structure (WBS), the project manager lists all required tasks for dependency analysis. Each task is then evaluated to determine what must happen before it can start or finish, and what must follow it.
A sequence diagram, often called a precedence diagramming method, is used to visualize these links, representing tasks as nodes and dependencies as directional arrows. Expert judgment, involving interviews with subject matter experts (SMEs), helps uncover non-obvious dependencies based on technical constraints. Gantt charts are used to visualize the schedule and confirm the logical flow of work.
Strategies for Managing and Mitigating Dependency Risk
Managing dependencies throughout the project lifecycle requires active monitoring and communication. For high-risk external dependencies, project managers must establish clear communication channels and constantly track the predecessor task status. Proactive communication with the external party helps manage expectations and allows for early warning of potential delays.
For internal dependencies, a common strategy is to build in buffers or slack time, known as float, into the schedule for non-Critical Path tasks. This float provides flexibility to absorb minor delays without impacting the overall project finish date. Project managers must also develop contingency plans for the most impactful dependencies, outlining an alternative course of action if a predecessor task is significantly delayed.