1. SWAG (Scientific Wild-Ass Guess)

• Description: A rough, experience-based guess that lacks detailed analysis.
• When to Use:
  • Early-stage discussions when little information is available.
  • When a quick ballpark figure is needed.
  • Informal or exploratory planning.
• Example:
  "This project might take around six months, but we need more details to be sure."

2. ROM (Rough Order of Magnitude)

• Description: A high-level estimate with a broad accuracy range (typically ±50% or more).
• When to Use:
  • Initial feasibility studies.
  • Budgeting and executive decision-making.
  • Before detailed scope or requirements are available.
• Example:
  "This project will cost between $500K and$1M."

3. T-Shirt Sizing (XS, S, M, L, XL, XXL, etc.)

• Description: Uses broad size categories instead of precise numbers, often based on complexity or effort.
• When to Use:
  • Agile development and backlog grooming.
  • When detailed effort estimation isn't practical.
  • Relative comparisons between tasks rather than absolute time estimates.
• Example:
  "Feature A is a Medium, but Feature B is an XL, so it will take significantly longer."

4. Three-Point Estimation (PERT: Program Evaluation and Review Technique)

• Description: Uses three estimates—Optimistic (O), Pessimistic (P), and Most Likely (M)—to calculate an expected value using the formula: $$E = \frac{O + 4M + P}{6}$$
• When to Use:
  • When uncertainty needs to be factored into planning.
  • For more accurate and risk-aware estimates.
• Example:
  "Best case: 3 days, worst case: 10 days, most likely: 6 days. Expected time = (3 + 4×6 + 10) ÷ 6 ≈ 6.2 days."

5. Story Points (Agile Estimation)

• Description: Assigns effort values to user stories based on complexity, risk, and unknowns, often using Fibonacci-like sequences (1, 2, 3, 5, 8, etc.).
• When to Use:
  • Agile teams working with Scrum or Kanban.
  • When estimating development effort without tying it directly to time.
• Example:
  "This user story is a 5 because it's more complex than a 3 but not as large as an 8."

Choosing the Right Estimation Method