Project Scheduling, Gantt Charts and the Critical Path

There is an old maxim in project management that captures the stakes of time management in four words: time is money. But in heavy engineering and defence, the reality is sharper than that. Time is contractual obligation. Time is liquidated damages. Time is the difference between a vessel entering service for a sovereign capability window and missing it entirely.

For most projects, if the work is completed to an acceptable level of quality, within an acceptable cost, and within acceptable time, almost all other faults can be overlooked. Get the schedule wrong, and nothing else matters.

This is Part 3 of our three-part Masterclass on the Planning Phase. In Mastering the Project Management Plan, we built the Project Management Plan. In Mastering the Work Breakdown Structure, we constructed the Work Breakdown Structure. Now we bring the project to life with time — sequencing activities, estimating durations, and building a schedule that can be monitored, controlled, and compressed when deadlines tighten.

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Why Time Management Is a Core Competency

The time management function is not simply about drawing a bar chart. It is a disciplined four-part system:

Core Principle: Without control, management does not exist. Control includes the recognition of what has been happening, what effect it will have, and what must be actioned. It must contain overt action — passive observation is monitoring, not control.

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The Six-Step Process for Building a Project Schedule

The PMBOK organises project time management into six sequential processes during the organising and preparing phase, followed by schedule control during execution.

1. Plan Schedule — Management

2. Define — Activities

3. Sequence — Activities

4. Estimate Activity — Resources

5. Estimate Activity — Durations

6. Develop — Schedule

7. Control — Schedule

Relationship details

From	Relationship	To
1. Plan Schedule — Management	leads to	2. Define — Activities
2. Define — Activities	leads to	3. Sequence — Activities
3. Sequence — Activities	leads to	4. Estimate Activity — Resources
4. Estimate Activity — Resources	leads to	5. Estimate Activity — Durations
5. Estimate Activity — Durations	leads to	6. Develop — Schedule
6. Develop — Schedule	leads to	7. Control — Schedule

Let's walk through each step.

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Step 1: Plan Schedule Management

Before creating the schedule itself, establish the policies, procedures, and documentation that will govern how the schedule is planned, developed, managed, executed, and controlled throughout the project.

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Step 2: Define Activities

This step identifies the specific schedule activities that must be performed to produce each project deliverable. The WBS from Part 2 is your starting point — you decompose work packages into the individual activities and tasks that will appear on the schedule.

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Step 3: Sequence Activities

With activities defined, the next step is to identify and document the dependencies between them. This is where the logical flow of the project emerges.

The Four Dependency Types

Finish-to-Start is by far the most common dependency type and the default assumption in most scheduling software.

Representing Dependencies: Two Methods

Arrow Diagramming Method (ADM) uses arrows to represent activities, connected at nodes to indicate dependencies:

Start

A

D

B

C

E

F

Finish

Relationship details

From	Relationship	To
Start	leads to	A
Start	leads to	D
A	leads to	B
B	leads to	C
D	leads to	E
E	leads to	F
C	leads to	Finish
F	leads to	Finish

Precedence Diagramming Method (PDM) — the modern standard — uses boxes (nodes) to represent activities, with arrows showing the dependencies:

START

A

D

B

C

E

F

FINISH

Relationship details

From	Relationship	To
START	leads to	A
START	leads to	D
A	leads to	B
B	leads to	C
D	leads to	E
E	leads to	F
C	leads to	FINISH
F	leads to	FINISH

Network planning techniques used by modern project management software — including Microsoft Project — are based on the PDM approach.

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Step 4: Estimate Activity Resources

For each schedule activity, estimate the type and quantities of resources required. Three elements must be identified and recorded:

1. Resources required for each activity (persons, equipment, or material)
2. Quantities of resources required for each activity
3. Availability — when each resource will be available, including any constraints

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Step 5: Estimate Activity Durations

This step estimates the number of work periods needed to complete each activity. After selecting an appropriate unit of time (days, weeks, or hours), allocate the duration for each resource and activity, taking into account:

• Periods when work is permitted (e.g., EPA noise restrictions, site access windows)
• Calendar types (normal business hours vs. multiple shifts)
• Public holidays (state and national)
• Team member leave and availability constraints

Three-Point Estimating (PERT Weighted Average)

For activities with significant uncertainty, the three-point estimate produces a statistically weighted duration:

$$T_e = \frac{T_o + 4T_m + T_p}{6}$$

Where:

Worked Example:

For an activity where the optimistic estimate is 1 week, the most probable is 2 weeks, and the pessimistic is 3 weeks:

$$T_e = \frac{1 + 4(2) + 3}{6} = \frac{1 + 8 + 3}{6} = \frac{12}{6} = 2.0 \text{ weeks}$$

The formula weights the most probable estimate four times more heavily than the optimistic or pessimistic, producing a result that accounts for uncertainty without being dominated by extreme scenarios.

Time Estimate →

Probability Density →

p — (Pessimistic)

o — (Optimistic)

m — (Most Likely)

Te — (Expected Time)

Relationship details

From	Relationship	To
o — (Optimistic)	leads to	C1
C1	leads to	C2
C2	leads to	C3
C3	leads to	C4
C4	leads to	C5
C5	leads to	C6
C6	leads to	p — (Pessimistic)
C3	leads to	m — (Most Likely)
C4	leads to	Te — (Expected Time)
Time Estimate →	leads to	o — (Optimistic)
Probability Density →	leads to	C3

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Step 6: Develop Schedule

This is the integration step — analysing activity sequences, durations, resource requirements, and schedule constraints to create the project schedule.

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Programming Techniques: From Simple to Complex

Milestone Chart

The simplest method, applicable to small projects. A milestone chart shows completion dates but does not show start dates or interrelationships between activities. It is useful for executive reporting but insufficient for day-to-day project management.

Gantt Chart (Bar Chart)

Designed by Henry Gantt during World War I, the bar chart remains the most widely used scheduling tool, particularly for small to medium projects. Most people can readily understand a Gantt chart, and while it doesn't easily show task interdependencies in its basic form, it does show overlapping activities clearly.

Building a Gantt Chart: The Seven-Step Process

Step 1: Set up new — project file

Step 2: Enter tasks — from WBS (scope)

Step 3: Enter durations — (time estimates)

Step 4: Enter predecessors — (sequencing)

Step 5: Enter resources — (HR management)

Step 6: Enter — estimated costs

Step 7: Save — baseline

Relationship details

From	Relationship	To
Step 1: Set up new — project file	leads to	Step 2: Enter tasks — from WBS (scope)
Step 2: Enter tasks — from WBS (scope)	leads to	Step 3: Enter durations — (time estimates)
Step 3: Enter durations — (time estimates)	leads to	Step 4: Enter predecessors — (sequencing)
Step 4: Enter predecessors — (sequencing)	leads to	Step 5: Enter resources — (HR management)
Step 5: Enter resources — (HR management)	leads to	Step 6: Enter — estimated costs
Step 6: Enter — estimated costs	leads to	Step 7: Save — baseline

Notice how each step maps directly to the concepts we have covered across this series: the WBS provides the tasks (Step 2), time estimates provide the durations (Step 3), dependency analysis provides the predecessors (Step 4), resource estimation provides the resource names (Step 5), and the cost management plan provides the estimated costs (Step 6).

Step 7 — saving the baseline — is critical. The baseline is the approved version of the schedule against which all future progress is measured. Without a baseline, you have no way to determine whether the project is ahead, behind, or on track.

Applied Example: Computer System Implementation

Using the Gantt chart activity from the course materials, consider this task list for a computer system implementation project:

The dependency structure reveals that Tasks 5, 6, and 8 all depend on the completion of Tasks 3 and 4 — meaning none of them can start until both hardware and software selection are finished. Since Task 4 takes 3 weeks (longer than Task 3's 2 weeks), the earliest start for Tasks 5, 6, and 8 is the end of Week 3.

Workstream	Activity	Type	Timing / dependency / duration
Initiation	Go-Ahead	Milestone	milestone · m1 · 2025-01-06 · 0d
Management	Manage Project	Activity	t2 · 2025-01-06 · 77d
Selection	Select Computers	Activity	t3 · 2025-01-06 · 14d
Selection	Select Software	Activity	t4 · 2025-01-06 · 21d
Procurement & Documentation	Purchase Computers/Software	Activity	t5 · after t3 t4 · 7d
Procurement & Documentation	Write Training Manual	Activity	t6 · after t3 t4 · 7d
Procurement & Documentation	Write User Manual	Activity	t7 · after t6 · 14d
Procurement & Documentation	Develop Training Program	Activity	t8 · after t3 t4 · 14d
Installation	Plan Installation	Activity	t9 · after t5 · 7d
Installation	Install Computers	Activity	t10 · after t9 · 14d
Installation	Install Software	Activity	t11 · after t10 · 21d
Training & Acceptance	Train Users	Activity	t12 · after t6 t8 t11 · 7d
Training & Acceptance	Dry Run / System Test	Activity	t13 · after t12 · 7d
Training & Acceptance	Acceptance	Milestone	milestone · m2 · after t13 t7 · 0d

Schedule Compression: From 12 to 11 Weeks

The activity asks how to compress this 12-week program to 11 weeks. The critical path runs through: Go-Ahead → Select Software (3 wk) → Purchase (1 wk) → Plan Installation (1 wk) → Install Computers (2 wk) → Install Software (3 wk) → Train Users (1 wk) → Dry Run (1 wk) → Acceptance = 12 weeks.

Compression options include:

The most practical approach is typically to crash the longest non-parallel critical-path activity (Install Software, at 3 weeks) by assigning additional technical resources.

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Network Planning (CPM and PERT)

For larger, more complex projects, the basic Gantt chart cannot adequately show the logical relationships between all project activities. In the 1950s, three network planning techniques were developed to address this limitation:

Critical Path Method (CPM) focuses on calculating float — the amount of time an activity can be delayed without affecting the project end date. Activities with zero float are on the critical path and cannot be delayed without extending the project.

Program Evaluation and Review Technique (PERT) emphasises milestones and uses the three-point weighted average estimate (the formula from Step 5) to incorporate uncertainty into the schedule.

Precedence Diagramming Method (PDM) uses nodes to represent activities with connecting arrows to show dependencies. This is the approach used by modern scheduling software.

Reconciling Timing Constraints

The final step in schedule development has three objectives:

1. Determine the anticipated duration of the entire project
2. Identify the activities which form the critical path — the longest sequence of dependent activities
3. Quantify the amount of float for all non-critical activities

Key Insight: The critical path determines the minimum possible project duration. Any delay to a critical-path activity delays the entire project by the same amount. Non-critical activities have "float" — spare time — and can absorb some delay without affecting the end date.

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What Scheduling Software Can and Cannot Do

This distinction is critical for any project manager relying on tools like Microsoft Project, Primavera, or similar platforms.

The Bottom Line: Software is a calculator, not a thinker. It processes the data you give it. If the WBS is incomplete, the dependencies are wrong, or the estimates are unrealistic, the software will produce a beautiful, precisely calculated, and completely misleading schedule.

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The Pitfalls: Where Scheduling Goes Wrong

1. Treating the Gantt chart as the plan. The schedule is one component of the Project Management Plan. Without scope, cost, quality, risk, and communications management, a schedule is a list of dates with no management infrastructure.

2. Not saving a baseline. Without a baseline, you cannot measure progress. You are tracking a moving target.

3. Using software before doing the thinking. Enter the project into scheduling software only after you have defined the WBS, identified dependencies, estimated durations, and assigned resources. The software automates the arithmetic — you do the engineering.

4. Ignoring float. Activities with float are opportunities for resource optimisation, risk mitigation, and cost savings. Treat float as a management tool, not wasted time.

5. Action-oriented bias. Many project managers feel they can handle any eventuality and tend to act first and think later. This reduces their effectiveness and, in turn, reduces the effectiveness of their entire project team.

6. Assuming the program is static. The schedule is a living document. It will be modified as the project proceeds and unanticipated changes in scope or timing occur. Build your schedule management processes to accommodate change, not resist it.

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Key Takeaways

• Time management has four sub-functions: planning, programming, monitoring, and control — all four must be active throughout the project.
• Follow the six-step PMBOK process to build a defensible schedule: Plan → Define → Sequence → Estimate Resources → Estimate Durations → Develop Schedule.
• Use the three-point PERT formula $$T_e = \frac{T_o + 4T_m + T_p}{6}$$ to estimate durations under uncertainty.
• Understand the four dependency types (FS, FF, SS, SF) — Finish-to-Start is the default and most common.
• The critical path determines minimum project duration — any delay to a critical-path activity delays the entire project.
• Always save a baseline — without it, progress measurement is impossible.
• Software is a calculator, not a decision-maker — the thinking must happen before the data entry.
• Schedule compression techniques (fast-tracking, crashing, overlapping) trade risk or cost for time — use them deliberately, not desperately.

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This article is part of the Principles of Project Management Masterclass Series. Content is aligned with the PMBOK® Guide framework and contextualised for heavy engineering, manufacturing, and defence project environments.