3.5 Breakdown-Structure Relationships and the Cost Estimate Model

3.5 Breakdown-Structure Relationships and the Cost Estimate Model

Integrated project control uses several structures. The WBS organizes scope, the OBS organizes responsibility, the CBS organizes cost, a Resource Breakdown Structure organizes resource types, and system or location structures support turnover and execution. The schedule must relate these views without pretending they are identical.

Use a code of accounts

A code of accounts supplies stable identifiers that connect records. An activity might carry codes for WBS work package, responsible organization, cost account, contract, area, system, phase, and resource class. These codes let one activity appear in different reports without duplicating it.

Control accounts

At a control account, defined scope, responsible management, budget, and schedule come together. Below it, work packages and activities provide detail. Above it, results aggregate for management.

The level should be controllable. A control account spanning unrelated systems and several managers weakens accountability; thousands of tiny accounts create administration without insight.

Map activities to scope

Every activity should support authorized scope or a necessary management/level-of-effort function. Mapping checks can identify:

• WBS elements with budget but no schedule work;
• activities with no WBS or contract basis;
• cost accounts that cannot collect actuals at a useful level;
• procurement or commissioning work missing from the construction-oriented WBS; and
• duplicated scope across contracts.

Not every WBS element requires a one-to-one activity. A work package may need several activities, and one summary activity may support early planning before detail is available. The mapping should be explainable and mature over time.

Inputs from the cost estimate model

The estimate informs scheduling through:

• quantities and work-package boundaries;
• crew composition and productivity;
• equipment and material needs;
• procurement packages and vendor lead times;
• direct and indirect cost assumptions;
• escalation and market constraints;
• allowances and risk assumptions; and
• cash-flow or funding limits.

The scheduler tests whether estimate assumptions produce feasible durations and resource demand. If 10,000 units are estimated at 100 units per crew-day with two crews, the unconstrained duration is about 50 crew-days of work, subject to calendars, learning, workfronts, and sequence.

Time-dependent and time-independent cost

Some cost changes with time: supervision, facilities, rentals, security, and certain project-management costs. Some is primarily quantity or purchase driven. The model should distinguish these when evaluating delay or acceleration. Extending an activity does not automatically multiply every material cost, and shortening it may increase labor premium or reduce time-dependent overhead.

Reconciliation

At baseline and forecast cycles, reconcile:

1. total authorized scope and budget;
2. control-account and work-package mapping;
3. schedule cost/resource loads to estimate values;
4. procurement milestones to commitments;
5. time-phased cash flow to forecast dates; and
6. changes across scope, cost, and schedule logs.

Differences may be legitimate—accounting may collect tax separately, for example—but they should be explained.

Example

A mechanical system is one WBS deliverable, two responsible organizations, three purchase orders, and four turnover subsystems. Activity coding can support each view. Creating four separate copies of the same installation activity to satisfy each hierarchy would double-count work and corrupt logic.

The objective is integrated traceability: one authorized scope, multiple useful views, and reconciled cost and time assumptions.

Applied review: integrate the organizing models

The WBS, OBS, CBS, responsibility assignments, and schedule coding structure are related views, not interchangeable labels. The WBS decomposes deliverables and work; the OBS identifies accountable organizational units; the CBS supports cost collection and control. Their intersections can define control accounts, work packages, and responsible managers. A coding dictionary and crosswalk let the schedule aggregate consistently without duplicating the same activity under multiple owners.

The cost estimate model supplies quantities, unit rates, crews, productivity, procurement allowances, indirect effort, and time-sensitive assumptions. Map those inputs to schedule work at a level where status and forecast decisions can be made. If one estimate line spans several phases or locations, allocate it using a documented basis rather than hiding it on a summary activity. Reconcile total mapped scope and cost to the authorized control basis.

Relationships among breakdown structures should support questions users actually ask: which deliverable is late, which organization owns the recovery, which account bears the cost, and which contract package supplies the work? Test the coding by producing these summaries and tracing selected totals back to detailed activities. In an exam scenario, neither the WBS nor the cost model automatically creates network logic. The planner still must identify activities, durations, interfaces, calendars, and relationships.