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We recommend reading these guides first to get the most out of this one:
EVMS Fundamentals
100%
Scope Rule
3–6
Typical Levels
WBS × OBS
= Control Accounts
881
MIL-STD Reference

Why WBS Comes First

You cannot plan, budget, schedule, or measure a program without first defining what the program contains. The WBS is that definition. Every other EVMS artifact depends on it: the schedule is organized by WBS. The budget is distributed by WBS. Earned value is measured by WBS. Cost is collected by WBS. If the WBS is wrong, everything built on it is wrong.

WBS Decomposition Rules

Rule 1: 100% Rule

Each level must capture 100% of the scope at that level. Nothing missing, nothing extra. The sum of all Level 3 elements must equal 100% of the Level 2 element they belong to.

Rule 2: Mutually Exclusive

No overlap between elements at the same level. If “Wing Assembly” includes sealant application, then “Fuselage Assembly” must not also include wing sealant. Double-counting scope means double-counting budget.

Rule 3: Outcome-Oriented

WBS elements should describe deliverables or outcomes, not activities. Not “Drilling” but “Wing Rib Assembly” (which includes drilling as one of many activities). Activities belong in the schedule; deliverables belong in the WBS.

Rule 4: Appropriate Depth

Decompose to the level where you can meaningfully plan, budget, and measure. Too shallow = elements too large to manage. Too deep = excessive administrative overhead. Typical aerospace programs: 3–6 levels, with the lowest level (work packages) representing 1–3 months of effort.

Aerospace WBS Example

📊 Fighter Aircraft WBS (Top 3 Levels) MIL-STD-881 Based
Level 1Level 2Level 3 (Examples)
1.0 Aircraft System1.1 Air Vehicle1.1.1 Airframe, 1.1.2 Propulsion, 1.1.3 Vehicle Subsystems
1.2 System Engineering1.2.1 Requirements Analysis, 1.2.2 System Design, 1.2.3 Integration & Test
1.3 Training1.3.1 Pilot Training, 1.3.2 Maintenance Training, 1.3.3 Training Devices
1.4 Support Equipment1.4.1 GSE, 1.4.2 Test Equipment
1.5 Data1.5.1 Technical Publications, 1.5.2 Engineering Data
1.6 Program Management1.6.1 PM Office, 1.6.2 Contract Admin, 1.6.3 Reviews
1.7 System Test & Evaluation1.7.1 DT&E, 1.7.2 OT&E

Level 1 is the total program. Level 2 is defined by MIL-STD-881 for each commodity type. Level 3 and below are contractor-defined based on program architecture and organizational structure.

WBS Dictionary

Every WBS element needs a dictionary entry that defines exactly what is included and excluded. Without the dictionary, two people looking at “1.1.1 Airframe” may have different assumptions about what it contains.

FieldContent
WBS Element1.1.1 Airframe
DescriptionDesign, fabrication, and assembly of the aircraft structural airframe including fuselage, wing, and empennage structures
IncludesStructural design, detail part fabrication, subassembly, final structural assembly, structural test articles
ExcludesAvionics installation (1.1.3), propulsion installation (1.1.2), systems integration (1.2.3)
Responsible OrgStructures Engineering + Manufacturing

WBS × OBS = Control Accounts

The Organizational Breakdown Structure (OBS) maps the performing organizations. The intersection of WBS (what work) and OBS (who does it) creates the control account — the point where scope, schedule, budget, and responsibility converge.

WBS
(What)
×
OBS
(Who)
→
Control
Account

Each control account has one CAM (Control Account Manager) who is accountable for the scope, schedule, and budget within that intersection. The CAM track covers this role in detail.

⚠️ WBS Mistakes That Cascade

Missing scope: If work is not in the WBS, it has no budget. When it is eventually discovered, it is funded from management reserve (if available) or creates an overrun. Wrong level: Elements that are too large cannot be meaningfully measured; elements that are too small create administrative burden that exceeds the value of the control. Activity-oriented: A WBS organized by activity (“Design,” “Build,” “Test”) makes it impossible to track the cost of a deliverable across its lifecycle.

🎯 The Bottom Line

The WBS is the skeleton of program management. It defines 100% of the scope, organizes it hierarchically, and — when crossed with the OBS — creates the control accounts where EVMS actually operates. Get the WBS right and everything downstream (budgeting, scheduling, earned value measurement, variance analysis) has a solid foundation. Get it wrong and no amount of sophisticated analysis can compensate. Next: Earned Value Metrics — the measurements that turn WBS structure into performance insight.

Keep Learning
📖
Guide
Earned Value Metrics
📖
Guide
CPI & SPI Analysis
📖
Guide
Work Package Planning
🎓 Part of the EVMS Reporting track — view all 8 guides →
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Take this to a room

The running order

For anyone building a programme baseline. They should leave able to test a WBS against the four rules and to explain how control accounts are formed.

7 beats · 11 min
  1. 1

    Nothing works without it

    The WBS is the skeleton. Budgeting, scheduling, earned value and variance analysis all hang off it.

    • Get it right and everything downstream has a foundation.
    • Get it wrong and no amount of sophisticated analysis compensates.
    • This is the cheapest thing to fix now and the most expensive later.

    Ask the room When was our WBS last checked against the scope?

  2. 2

    The 100 per cent rule

    Each level captures all of the scope at that level. Nothing missing, nothing extra.

    • The sum of the level 3 elements equals 100 per cent of their level 2 parent.
    • If work is not in the WBS, it has no budget.
    • When it is discovered later it comes from management reserve, or it becomes an overrun.
  3. 3

    Mutually exclusive

    No overlap between elements at the same level, because overlap is double-counted budget.

    • If Wing Assembly includes sealant, Fuselage Assembly must not also include wing sealant.
    • Double-counting scope means double-counting budget.
    • It also means two managers both think someone else is doing it.
  4. 4

    Deliverables, not activities

    This is the rule most often broken, and the damage is not obvious until measurement starts.

    • Not Drilling. Wing Rib Assembly, which includes drilling among many activities.
    • Activities belong in the schedule. Deliverables belong in the WBS.
    • An activity-oriented WBS makes earned value nearly impossible to claim objectively.

    Ask the room Are our level-four elements nouns or verbs?

  5. 5

    The right depth

    Decompose to where you can meaningfully plan, budget and measure. Not further.

    • Too shallow: elements too large to manage or measure.
    • Too deep: administrative overhead exceeds the value of the control.
    • Typically three to six levels for an aerospace programme.
  6. 6

    The dictionary is not optional

    The WBS names the elements. The dictionary says what is inside each one.

    • Scope description, deliverables, what is explicitly excluded.
    • Without it, two people read the same element name differently.
    • Most scope disputes are dictionary disputes that nobody wrote down.
  7. 7

    WBS crossed with OBS gives control accounts

    What crossed with who. That intersection is where EVMS actually operates.

    • Every control account has one accountable manager.
    • That is the level where budget, schedule and measurement meet.
    • If you cannot draw that grid, you do not yet have a baseline.

    Ask the room Can we draw our WBS-OBS grid on a whiteboard right now?

Continue the EVMS Track

📊 Earned Value Metrics
PV, EV, AC and every derived metric
📈 CPI & SPI Analysis
Performance indices and trend analysis
📋 Work Package Planning
Planning at the control account level