Ch3. Scope, Schedule, and Cost Management — The Triple Constraint

The Triple Constraint

All projects operate within three interrelated constraints:

        Scope
          △
         / \
        /   \
       /     \
 Schedule ──── Cost

Changing one affects the others. The PM’s job is to manage this balance.

Scope Management

WBS (Work Breakdown Structure)

Hierarchical decomposition of the total project work into manageable components.

Project
├── 1. Planning
│   ├── 1.1 Requirements gathering
│   └── 1.2 Scope definition
├── 2. Development
│   ├── 2.1 Backend
│   └── 2.2 Frontend
└── 3. Testing
    └── 3.1 QA

Work packages: The lowest level WBS elements — small enough to estimate schedule and cost.

Scope Creep

Gradual, unauthorized expansion of scope without formal approval. A leading cause of project failure.

Prevention: Formal change control process, documented scope baseline, clear stakeholder communication.

Exam answer pattern: When a stakeholder requests a new feature informally, the correct PM action is always to direct them through the formal Change Control Process — not to immediately accept or reject the request.

Schedule Management: Critical Path Method (CPM)

Calculates the minimum project duration and identifies which activities have schedule flexibility.

Critical path: The longest sequence of activities = minimum project duration.

Activity A (3 days) → Activity C (2 days)
                                              → End (CPM = 8 days)
Activity B (5 days) → Activity D (3 days)

Critical path: B → D (8 days) Path A → C = 5 days → Float = 3 days (can be delayed up to 3 days without delaying the project)

Float (slack): How long an activity can be delayed without impacting the end date.

Activities on the critical path have Float = 0

Cost Management: Earned Value Management (EVM)

Integrates scope, schedule, and cost data to provide an objective view of project performance.

Core Metrics

Metric	Formula	Meaning
PV	Planned Value	Budgeted value of work planned to be done by now
EV	Earned Value	Budgeted value of work actually completed
AC	Actual Cost	Actual amount spent so far

Variances and Indices

Metric	Formula	Interpretation
CV	EV – AC	Positive=under budget, Negative=over budget
SV	EV – PV	Positive=ahead of schedule, Negative=behind
CPI	EV / AC	>1=efficient, <1=over-spending
SPI	EV / PV	>1=ahead, <1=behind

Example:

Mid-project checkpoint:
PV = $500K (planned to complete $500K of work by now)
EV = $400K (only $400K of work actually completed)
AC = $480K (spent $480K)

CV = 400 - 480 = -80K (over budget)
SV = 400 - 500 = -100K (behind schedule)
CPI = 400/480 = 0.83 (generating $0.83 of value per dollar spent)
SPI = 400/500 = 0.80 (80% of planned progress achieved)

Key Concept Cards

WBS ★★★★★ : Hierarchical scope decomposition. Foundation for schedule and cost estimation. No work should be missing.

Critical Path ★★★★★ : Activities with Float=0 define the project’s minimum duration. Any delay here delays the project.

CPI and SPI ★★★★★ : Above 1.0 = good, below 1.0 = problem. CPI=cost efficiency, SPI=schedule efficiency. Core EVM metrics.

Practice Quiz

Q1. A project has CPI = 0.8 and SPI = 1.2. What does this mean?

The project is ahead of schedule (SPI > 1) but over budget (CPI < 1). Work is being completed faster than planned, but more money is being spent than budgeted to accomplish it. The PM must investigate cost overruns and update the cost forecast (EAC).

Q2. What is the most effective way to prevent scope creep?

Establish and enforce a formal Change Control Process. All scope changes must go through this process — no informal additions. Maintain a clear, documented scope baseline; educate stakeholders on the process; and conduct regular scope reviews. Documentation is defense against unauthorized changes.