Stacey Matrix
Model · Project Complexity
Stacey Matrix
Two-axis model for classifying projects by What / Requirements (clear ↔ unclear) and How / Approach (known ↔ untested). Determines the appropriate PM method.
🟢 Simple
What & How are clear and known
→ Standard approach, predictable flow
🔵 Complicated
Causality is linear, but combination needed
→ Classical PM + complementary methods (e.g. Lean)
🟡 Complex
Unpredictable flow, many interdependencies & risks
→ Agile methods — high flexibility required
🔴 Chaotic
Edge of innovation, barely plannable
→ Goal: move into "complex" zone
Key insight: The higher the complexity according to the Stacey Matrix, the harder the effort estimation — and the more agile methods have the advantage.
Planning — Sequence & Effort
Concept
Sequence Plan
Puts the WBS elements into an actionable order.
- Prerequisite: dependencies between work packages known
- Foundation for schedule and resource planning
- Key questions: Which WPs must finish first? Which run in parallel?
- WBS shows What — Sequence Plan shows In what order
Concept
Effort Estimation
- Created for each element of the sequence plan
- The more complex (→ Stacey), the harder the estimation
- No exact method — best done by experts
- Result feeds directly into the project schedule
- Typical unit: weeks per work package
Overview
Planning Chain at a Glance
WBSWhat are the work packages?
→
Sequence PlanIn what order?
→
Effort EstimationHow long does each WP take?
→
Project ScheduleWhen exactly? With buffer & milestones
Project Schedule & Critical Path
Concept
Project Schedule
Derived from the project time horizon, sequence plan and effort estimation.
- Shows each activity with a concrete start and end date
- Does not include a view of available resources
- Constraints to consider: calendar, holidays, steering committees
- Formats: Network diagram or Gantt chart
Concept
Buffer / Float
Time allowance for delays without impact on subsequent activities or the project end.
- Protects the schedule against unforeseen delays
- Activities without buffer lie on the critical path
- Deliberately planning buffer = good project management
Core Concept
Critical Path
The longest sequence of activities that determines the project end. WPs on it have TF = 0 (no total float).
⚠ On the critical path
- TF = 0 — any delay endangers the project end
- Requires highest management attention
- Assign resources with priority
✓ Off the critical path
- TF > 0 — delay up to float length is tolerable
- Provides flexibility in resource allocation
- Can be treated as lower priority
Formats: Network diagram shows dependencies and critical path explicitly (with ES/EF/LS/LF/TF). Gantt chart shows the time axis and parallelism more intuitively — both complement each other.
Example · Activity-on-Node Diagram
Network Diagram — Example Project (simplified)
Critical Path (TF=0) Non-critical (TF>0) ◆ Milestone
Example · Project Schedule
Gantt Chart — Example Project (simplified)
Critical Path Non-critical Float ◆ Milestone
Milestones
Concept
Milestones
Defined points in the project timeline that mark the completion of an important phase or deliverable.
Requirements for Milestones
- Clearly and unambiguously defined
- Achievement must be verifiable beyond doubt (yes/no)
- Always assigned to a work package
- Basis for progress measurement & controlling
Typical Milestones (Example)
- Analysis completed
- Concept approved by client
- Pilot successfully signed off
- Project closure / go-live
Why it matters: Without clearly measurable milestones, project progress cannot be assessed objectively — steering & controlling becomes unreliable.
Exercise
Group Exercise — Example Project
IT Security project: training, awareness & monitoring. Deliverables per group:
Task 1
6–8 Work Packages
Orientation: Analysis → Concept → Execution → Test/Pilot → Closure
Task 2
Estimate Duration
Rough effort estimate per WP in weeks
Task 3
Dependencies
Which WPs depend on each other? Which can run in parallel?
Task 4
Network Diagram
Graphical representation with WP numbers
Task 5
Project Schedule
Tabular or as a Gantt chart
Task 6
4 Milestones + critical path
Clearly defined, assigned to a WP
How It All Connects
Synthesis
How It All Connects
01
The Stacey Matrix determines upfront how structured or agile the project should be managed — directly influencing the sequence plan and effort estimation.
02
WBS → Sequence Plan → Effort Estimation → Project Schedule is the classical planning chain: each document builds on the previous one.
03
The critical path emerges from the project schedule and shows which WPs have zero float — that's where management effort is highest.
04
Milestones are the measurable checkpoints in the schedule that make steering & controlling possible in the first place.
05
Projects with unclear requirements and an untested approach rank as complex on the Stacey Matrix → argues for an agile or hybrid approach.
Exam Preparation
Formulas · memorise these
Network Diagram Formulas
Earliest Finish
EF = ES + D
Latest Start
LS = LF − D
Total Float
TF = LS − ES
Forward pass: ES = 0 for the first WP; ES of successor = max(EF of all predecessors). Backward pass: LF of last WP = EF of project end; LF of predecessor = min(LS of all successors).
Definitions · memorise these
Key Terms at a Glance
Stacey Matrix
Model for classifying projects by the clarity of requirements (What) and the familiarity of the approach (How) → determines the appropriate PM method.
Sequence Plan
Puts WBS elements into an actionable order. Shows dependencies: sequential vs. parallel. Foundation for the project schedule.
Critical Path
Longest sequence of dependent activities in the project. All WPs on this path have TF = 0. Any delay shifts the project end.
Total Float (TF)
TF = LS − ES. Indicates how many time units a WP can be delayed without endangering the project end date.
Milestone
Defined point in the project timeline marking the completion of a phase. Must be unambiguous and verifiable beyond doubt. Always assigned to a WP.
Project Schedule
Derived from time horizon, sequence plan and effort estimation. Shows concrete start and end dates. Contains no resource view.
ES / EF
Earliest Start and Earliest Finish — from the forward pass: EF = ES + D.
LS / LF
Latest Start and Latest Finish — from the backward pass: LS = LF − D.
Strategy
Exam Tips
Know your formulas cold
EF = ES + D, LS = LF − D, TF = LS − ES. These three formulas cover all network diagram calculations. Always state the unit (weeks).
Justify your Stacey placement
For Stacey Matrix questions: name both axes, state the classification AND explain why → then state which method follows from it.
Mark the critical path explicitly
In the network diagram, identify all WPs with TF=0 and explicitly label them as the critical path. Total duration = sum of durations on the critical path.
Milestone ≠ Task
"Concept created" is a task. "Concept approved by client" is a milestone — a measurable checkpoint, not an activity.
Memorise the planning chain
WBS → Sequence Plan → Effort Estimation → Project Schedule. Know this order and why each step depends on the previous one — it can be asked directly.
Network diagram vs. Gantt
Network diagram = dependencies & float visible. Gantt = time axis & parallelism more intuitive. If asked: name both and explain the difference.
Practice questions · click to reveal
Possible Exam Questions
What do the two axes of the Stacey Matrix show, and what do they determine? ▶
The X-axis shows How / Approach (known ↔ untested), the Y-axis shows What / Requirements (clear ↔ unclear). The combination determines whether a project is simple, complicated, complex or chaotic — and therefore which PM method (classical, agile, hybrid) is appropriate.
What is the difference between a WBS and a sequence plan? ▶
The WBS lists what needs to be done (work packages, deliverables). The sequence plan puts these into an order: in what sequence and which WPs can run in parallel. Only from the sequence plan can a project schedule be created.
What is the critical path and how do you identify it? ▶
The critical path is the longest chain of dependent activities — it determines the minimum project duration. It is identified via the network diagram: all WPs with
TF = 0 lie on the critical path. Any delay on this path delays the entire project.Explain forward pass and backward pass in a network diagram. ▶
Forward pass (earliest dates): Start at the first WP with ES = 0.
Backward pass (latest dates): Start at the last WP: LF = EF of project end.
Total Float:
EF = ES + D. ES of successor = max(EF of all predecessors).Backward pass (latest dates): Start at the last WP: LF = EF of project end.
LS = LF − D. LF of predecessor = min(LS of all successors).Total Float:
TF = LS − ES.
What requirements must a milestone fulfil? ▶
A milestone must:
- Be clearly and unambiguously defined
- Be verifiable beyond doubt (yes/no — not "80% reached")
- Be assigned to a specific work package
- Serve as a basis for progress measurement and controlling
What does a project schedule contain — and what explicitly not? ▶
A project schedule contains: concrete start and end dates for each activity, milestones, float, and a representation of the critical path (Gantt or network diagram).
It does not contain a resource view — i.e. who does the task or with what budget. That is the job of resource planning.
It does not contain a resource view — i.e. who does the task or with what budget. That is the job of resource planning.
Why might a project with unclear requirements be better suited to agile PM? ▶
According to the Stacey Matrix: if requirements (What) are partially unclear and the approach (How) is partially untested, the project sits in the complex zone → agile or hybrid methods provide the flexibility needed to adjust course as the project progresses.
What is the difference between a network diagram and a Gantt chart? ▶
Network diagram: Explicitly shows dependencies between WPs, ES/EF/LS/LF/TF values and the critical path. Best for schedule calculations.
Gantt chart: Shows the time axis and parallelism of WPs more intuitively, but dependencies and float values are less directly readable.
Both complement each other — in practice both are often used together.
Gantt chart: Shows the time axis and parallelism of WPs more intuitively, but dependencies and float values are less directly readable.
Both complement each other — in practice both are often used together.