8.1 Scheduling & the Critical Path Method
Key Takeaways
- The Critical Path Method (CPM) identifies the longest sequence of dependent activities; activities on the critical path have zero float and determine the minimum project duration
- Float (slack) is the time an activity can slip without delaying the project; it equals late start minus early start (or late finish minus early finish)
- A schedule of values breaks the contract price into billable line items that drive progress billing and must agree with the project schedule
- A Gantt chart plots activities against a time scale but does not by itself show logical dependencies between activities
- To find the critical path, perform a forward pass to compute early start/finish and a backward pass to compute late start/finish; the path where early and late values match is critical
Why Scheduling Matters
Project scheduling is the backbone of every construction project. A schedule communicates the sequence and timing of activities, assigns responsibility, and gives the owner a basis to monitor progress. For the Arizona contractor, scheduling competence directly affects profitability, claims defense, and the ability to coordinate subcontractors across trades.
Quick Answer: The Critical Path Method (CPM) identifies the longest sequence of dependent activities in a project; activities on that critical path have zero float, so any delay to them delays the entire project.
Gantt Charts
A Gantt chart is a horizontal bar chart that plots activities against a time scale. Each bar shows when an activity starts and ends. Gantt charts are simple to read and useful for communicating overall progress to owners and subs, but they do not by themselves show the logical dependencies between activities — you cannot tell from a Gantt bar alone whether one activity must follow another or simply is planned to follow it. That dependency information is what CPM adds.
The Schedule of Values
A schedule of values is a detailed breakdown of the contract price into assignable portions of work. For a residential foundation project it might look like:
| Line Item | Scheduled Value |
|---|---|
| Sitework | $24,000 |
| Concrete | $28,000 |
| Masonry | $15,000 |
| Framing | $45,000 |
Each line item drives progress billing. Applications for payment reference the schedule of values to show percent complete per line. A realistic schedule and an accurate schedule of values must agree — if the schedule front-loads work that the schedule of values pays poorly, cash flow suffers and the contractor funds the owner's project.
The Critical Path Method (CPM)
The Critical Path Method (CPM) is a network-based scheduling technique that identifies the longest sequence of dependent activities through a project. That longest path sets the minimum total project duration. Activities on the critical path have zero float — any delay to a critical-path activity delays the whole project.
Key CPM Terms
| Term | Meaning |
|---|---|
| Predecessor | An activity that must finish before another can start |
| Successor | An activity that cannot start until a predecessor finishes |
| Float (slack) | The time an activity can slip without delaying project completion |
| Critical path | The chain of activities with zero float; the longest dependent path |
| Early start / early finish | The soonest an activity can begin and end based on predecessors |
| Late start / late finish | The latest an activity can begin and end without delaying the project |
Finding the Critical Path
To find the critical path, perform two passes through the activity network:
- Forward pass — start at the first activity (early start = 0) and add each activity's duration to compute its early finish. Each successor's early start equals the maximum early finish of all its predecessors. The project's earliest completion is the maximum early finish of the last activity.
- Backward pass — start at the project end (late finish = earliest completion) and subtract each activity's duration to compute its late start. Each predecessor's late finish equals the minimum late start of all its successors.
The path where early start equals late start (and early finish equals late finish) for every activity is the critical path. Float for any activity equals late start minus early start.
A Worked CPM Example
Consider a small residential foundation with five activities:
| Activity | Duration (days) | Predecessor |
|---|---|---|
| A — Excavate | 2 | — |
| B — Formwork | 3 | A |
| C — Pour concrete | 1 | B |
| D — Backfill | 2 | A |
| E — Strip forms | 1 | C, D |
Forward pass:
- A: ES=0, EF=2
- B: ES=2, EF=5
- C: ES=5, EF=6
- D: ES=2, EF=4
- E: ES=max(C EF, D EF)=6, EF=7
Backward pass:
- E: LF=7, LS=6
- C: LF=6, LS=5
- D: LF=6, LS=4
- B: LF=5, LS=2
- A: LF=2, LS=0
The longest path is A → B → C → E at 7 days. Path A → D → E is only 5 days, so D has 2 days of float (LS 4 − ES 2 = 2). Activities A, B, C, and E are on the critical path with zero float.
In the diagram, the orange (critical) activities form the longest chain; the blue activity has float and can slip up to 2 days without affecting the project end date.
Why CPM Matters on the Exam and the Job
The PSI trade exam expects you to read small activity networks, identify which activities are critical, and compute float by subtraction. On the job, CPM supports delay claims, change-order pricing, and recovery planning — if a non-critical activity slips within its float, no recovery is needed; if a critical activity slips, you must crash it (add resources) or fast-track it (overlap sequential activities) to recover the lost time.
Schedule Compression
When the critical path slips, two compression techniques apply:
- Crashing — adding labor, equipment, or overtime to shorten a critical activity, usually at higher cost
- Fast-tracking — re-sequencing activities that were planned in series to run in parallel, which increases risk and rework exposure
Both techniques trade cost or risk for time and should be documented through the change-order process.
In the Critical Path Method, what defines the critical path?
An activity has early start day 5, early finish day 9, late start day 8, and late finish day 12. What is its float?