How do you Estimate Project Effort? - Google Project Mgmt
Google Project Management Interview Question and Answers - How do you estimate project effort?
Guiding Principles
Start with the product (deliverables), not tasks. Estimates should trace to WBS/work packages.
Use historical data first. Calibrate methods with team productivity and past performance.
Triangulate techniques. Don’t rely on a single method — use bottom-up, parametric and analogous together.
Make assumptions explicit. Record them in the Basis of Estimate (BoE).
Provide ranges and confidence levels. Stakeholders get a point estimate + range + confidence.
Treat estimates as living. Re-estimate as you gain information (progressive elaboration).
Techniques:
Bottom-up estimating: Decompose WBS to work packages, estimate each, roll up. Best for accuracy.
Analogous estimating: Use similar past projects as reference (fast, lower precision).
Parametric estimating: Use metrics (e.g., hours per API, LOC, function points) × quantity.
Three-point (PERT): Optimistic / Most-Likely / Pessimistic → expected value E = (O + 4M + P) / 6.
Wideband Delphi / Planning Poker: Consensus methods for teams (good for story points).
T-shirt sizing / story points: Relative sizing for Agile; convert to time using historical velocity.
Function points / COSMIC: For sizing software where available.
Risk-adjusted estimating: Add expected risk exposure (probability × impact) to base estimate.
Expert judgment: Use SMEs when historical data is not available.
Step-by-Step Process
Step 1: Prepare
Confirm WBS / deliverables and acceptance criteria.
Gather historical data (velocity, productivity, past estimate accuracy).
Identify available resources & calendars (holidays, FTE% allocations).
List known risks and constraints.
Step 2: Estimate at the work package level
Use bottom-up where accuracy matters: for each work package, get SME estimate (hours/days).
For repeated tasks, use parametric formulas (e.g., “API = 8 hrs each”).
For features in Agile, use story points via planning poker/wideband Delphi.
Step 3: Aggregate & adjust
Roll up estimates to control accounts and the project level.
Add resource overhead (meetings, non-project time), productivity factors (context switching), and any tool/setup time.
Step 4: Add contingency & reserves
Compute contingency from quantified risks or historical variance.
Keep management reserve for unknowns (governed by sponsor/PMO).
Step 5: Review & validate
Peer review and independent estimate check (cross-team or PMO).
Reconcile differences (triangulate analogical, parametric, and bottom-up).
Document BoE, assumptions, confidence level, and scenarios (best/worst/most-likely).
Step 6: Present & get buy-in
Present point estimate, range, underlying assumptions, top risks, and recommended contingency.
Agree on acceptance criteria for re-estimation and who signs off.
Artifacts
Estimate workbook (work package line items, hours, owners).
Basis of Estimate (BoE) — assumptions, method used, historical references.
Contingency & reserve summary (risk exposure calc).
Estimate confidence & sensitivity analysis.
Estimate sign-off sheet.