Developing a project schedule is the process of defining exactly when the work defined in the Work Breakdown Structure (WBS) will be performed. The schedule translates the project scope into a timeline, creating a roadmap that guides execution and allows stakeholders to track progress.
A well-built schedule is not just a list of target dates; it's a dynamic model that responds to changes. When one task is delayed, a good schedule automatically recalculates the impact on dependent tasks and the final completion date.
The Scheduling Process
Building a robust schedule involves a logical sequence of steps, moving from work packages to a complete timeline:
Key Concepts in Scheduling
Dependencies (Relationships)
Tasks rarely happen in isolation; they depend on one another. The four types of logical relationships are:
- Finish-to-Start (FS): Task A must finish before Task B can start. (Most common: Write code → Test code)
- Start-to-Start (SS): Task A must start before Task B can start. (Pour concrete → Start leveling)
- Finish-to-Finish (FF): Task A must finish before Task B can finish. (Writing document → Editing document)
- Start-to-Finish (SF): Task A must start before Task B can finish. (Rare: New shift starts → Old shift finishes)
Leads and Lags
- Lead: Accelerating a successor task (e.g., starting testing before coding is 100% complete).
- Lag: Delaying a successor task (e.g., waiting 3 days for paint to dry before hanging pictures).
Milestones
Milestones are zero-duration points in time that represent a significant event, such as a phase completion, a key approval, or a major deliverable handoff. They are crucial for executive reporting, as sponsors usually care about milestones, not individual tasks.
The Critical Path
The Critical Path Method (CPM) is arguably the most important concept in predictive scheduling. The critical path is the longest sequence of dependent activities in the project network. It determines the shortest possible duration to complete the project.
- Any delay on the critical path directly delays the project's finish date.
- Tasks on the critical path have zero float (or slack).
- Tasks not on the critical path have float — they can be delayed up to a certain point without delaying the overall project.
As a project manager, you focus your attention—and your best resources—on the critical path.
Schedule Compression
If the calculated schedule runs past the required deadline, you must compress it. There are two primary techniques:
Adding more resources to critical path tasks to finish them faster (e.g., approving overtime, hiring contractors).
Trade-off: Increases cost. May also increase risk if new resources require onboarding time (Brooks' Law).
Overlapping activities that were originally planned to run sequentially (e.g., starting development before design is fully approved).
Trade-off: Increases risk. Operating in parallel often leads to rework if upstream tasks change.
Common Mistakes
- Mandating dates top-down: When management dictates the finish date before the work is estimated, the schedule becomes a work of fiction rather than a predictive model.
- Ignoring resource availability: Estimating an activity at 40 hours of effort and scheduling it to be done in one week assumes the resource is 100% dedicated to that task. In reality, meetings, admin, and other projects reduce actual availability.
- Hard-coding dates: Manually typing in start/finish dates instead of linking tasks with dependencies. If one task shifts, the rest of the schedule won't automatically update.
- Forgetting sick leave and holidays: A common oversight that immediately makes the schedule inaccurate.