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.
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."