mathews-tom/estimate-calibrator

Estimate Calibrator

Replaces single-point guesses with structured three-point estimates: decomposes work into atomic units, estimates best/likely/worst case for each, identifies unknowns and assumptions, calculates aggregate ranges using PERT, and assigns confidence levels with explicit rationale.

Reference Files

| File | Contents | Load When | | ---------------------------------- | ------------------------------------------------------------------------- | ---------------------- | | references/estimation-methods.md | PERT formula, three-point estimation, Monte Carlo basics | Always | | references/unknown-categories.md | Technical, scope, external, and organizational uncertainty types | Unknown identification | | references/calibration-tips.md | Cognitive biases in estimation, historical calibration, buffer strategies | Always | | references/sizing-heuristics.md | Common task size patterns, complexity indicators, reference class data | Quick sizing needed |

Prerequisites

• Work item description (feature, task, project)
• Decomposed tasks (or use task-decomposer skill first)
• Context: team familiarity, tech stack, existing codebase

Workflow

Phase 1: Decompose Work

If the work item is not already decomposed into atomic units:

1. Break into tasks — Each task should be estimable independently.
2. Right granularity — Tasks should be 1 hour to 3 days. Larger tasks have higher uncertainty; break them down further.
3. Identify dependencies — Tasks on the critical path determine the minimum duration.

Phase 2: Three-Point Estimate

For each task, estimate three scenarios:

| Scenario | Definition | Mindset | | ----------- | --------------------------------------- | --------------------------------------------- | | Best case | Everything goes right. No surprises. | "If I've done this exact thing before" | | Likely case | Normal friction. Some minor obstacles. | "Realistic expectation with typical setbacks" | | Worst case | Significant problems. Not catastrophic. | "Murphy's law but not a disaster" |

Key rule: Worst case is NOT "everything goes wrong." It's the realistic bad scenario (90th percentile), not the apocalyptic one (99th percentile).

Phase 3: Identify Unknowns

Categorize unknowns that affect estimates:

| Category | Example | Impact | | -------------- | ------------------------------------- | -------------------------------------------- | | Technical | "Never used this library before" | Likely case inflated, worst case much higher | | Scope | "Requirements may change" | All estimates may shift | | External | "Depends on API access from partner" | Blocking risk — could delay entirely | | Integration | "Haven't tested with production data" | Hidden complexity at integration | | Organizational | "Need design approval" | Calendar time, not effort time |

Phase 4: Calculate Ranges

For individual tasks, use the PERT formula:

Expected = (Best + 4 × Likely + Worst) / 6
Std Dev = (Worst - Best) / 6

For aggregate (project) estimates:

• Sum of expected values for total expected duration
• Root sum of squares of std devs for aggregate uncertainty

Phase 5: Assign Confidence

| Confidence | Meaning | When | | ---------- | -------------------------------- | --------------------------------------------- | | High | Likely case within ±20% | Well-understood task, team has done it before | | Medium | Likely case within ±50% | Some unknowns, moderate familiarity | | Low | Likely case within ±100% or more | Significant unknowns, new technology |

Output Format

## Estimate: {Work Item}

### Summary
| Scenario | Duration |
|----------|----------|
| Best case | {time} |
| Likely case | {time} |
| Worst case | {time} |
| **PERT expected** | **{time}** |
| **Confidence** | **{High/Medium/Low}** |

### Task-Level Estimates

| # | Task | Best | Likely | Worst | PERT | Unknowns |
|---|------|------|--------|-------|------|----------|
| 1 | {task} | {time} | {time} | {time} | {time} | {key unknown or "None"} |
| 2 | {task} | {time} | {time} | {time} | {time} | {key unknown} |
| | **Total** | **{sum}** | **{sum}** | **{sum}** | **{pert}** | |

### Key Unknowns

| # | Unknown | Category | Impact on Estimate | Mitigation |
|---|---------|----------|-------------------|------------|
| 1 | {unknown} | {Technical/Scope/External} | +{time} if realized | {spike, prototype, early test} |

### Assumptions
- {Assumption 1 — what must be true for this estimate to hold}
- {Assumption 2}

### Risk Factors
- {Risk}: If realized, adds {time}. Likelihood: {High/Medium/Low}.

### Confidence Rationale
**{High/Medium/Low}** because:
- {Specific reason — e.g., "Team has built 3 similar features"}
- {Specific reason — e.g., "External API is a new integration"}

### Recommendation
{Commit to PERT expected with {X}% buffer, or spike the top unknown first.}

Calibration Rules

1. Three points, not one. Single-point estimates are always wrong. Three points communicate uncertainty — the most important part of any estimate.
2. Worst case is the 90th percentile, not the 99th. "Asteroid hits the office" is not a useful worst case. "The API documentation is wrong and we need to reverse-engineer the protocol" is realistic worst case.
3. Unknowns inflate estimates more than known difficulty. A hard but well-understood task is more predictable than an easy but novel one.
4. Estimates are not commitments. Communicate ranges, not deadlines. If stakeholders need a single number, give the PERT expected plus a buffer for confidence level.
5. Spike unknowns early. If a single unknown dominates the estimate range, invest 1-2 days spiking it before estimating the rest.

Error Handling

| Problem | Resolution | | ------------------------------------- | --------------------------------------------------------------------------------------------------- | | Work item not decomposed | Decompose into 3-8 tasks first (or suggest task-decomposer skill). | | No historical reference | Estimate relative to a known task: "This is about 2x the auth feature." | | Stakeholder wants a single number | Provide PERT expected with buffer matching confidence level (High: +20%, Medium: +50%, Low: +100%). | | Estimate seems too large | Check for scope creep in task list. Remove non-essential tasks. Identify what can be deferred. | | Team has never done this type of work | Mark confidence as Low. Recommend a spike before committing to an estimate. |

When NOT to Estimate

Push back if:

• The work is exploratory (research, spikes) — timebox instead of estimating
• Requirements are completely undefined — define scope first
• The user wants precision (hours) for a large project — provide ranges, not false precision
• The estimate will be used as a commitment without acknowledging uncertainty