Policyengine Microsimulation Skill

---
name: policyengine-microsimulation
description: |
  ALWAYS USE THIS SKILL for PolicyEngine microsimulation, population-level analysis, winners/losers calculations.
  Triggers: "microsimulation", "share who would lose/gain", "policy impact", "national average", weighted analysis.
  Use this skill's code pattern, but explore the codebase to find specific parameter paths if needed.
---

# PolicyEngine Microsimulation

## Documentation References

- **Microsimulation API**: https://policyengine.github.io/policyengine-us/usage/microsimulation.html
- **Parameter Discovery**: https://policyengine.github.io/policyengine-us/usage/parameter-discovery.html
- **Reform.from_dict()**: https://policyengine.github.io/policyengine-core/usage/reforms.html

## CRITICAL: Use calc() with MicroSeries - No Manual Weights Ever

**MicroSeries handles all weighting automatically. Never access .weights or do manual weight math.**

```python
# ✅ CORRECT - MicroSeries handles everything
change = reformed.calc('household_net_income', period=2026, map_to='person') - \
         baseline.calc('household_net_income', period=2026, map_to='person')
loser_share = (change < 0).mean()  # Weighted automatically!

# ❌ WRONG - never access .weights or do manual math
loser_share = change.weights[change.values < 0].sum() / change.weights.sum()
```

## Quick Start

```python
from policyengine_us import Microsimulation
from policyengine_core.reforms import Reform

baseline = Microsimulation()
reform = Reform.from_dict({
    'gov.irs.credits.ctc.amount.base[0].amount': {'2026-01-01.2100-12-31': 3000}
}, 'policyengine_us')
reformed = Microsimulation(reform=reform)

# calc() returns MicroSeries - all operations are weighted automatically
baseline_income = baseline.calc('household_net_income', period=2026, map_to='person')
reformed_income = reformed.calc('household_net_income', period=2026, map_to='person')
change = reformed_income - baseline_income

# Weighted stats - no manual weight handling needed!
print(f"Average impact: ${change.mean():,.0f}")
print(f"Total cost: ${-change.sum()/1e9:,.1f}B")
print(f"Share losing: {(change < 0).mean():.1%}")
```

## Available Datasets (HuggingFace)

```python
# National (default)
sim = Microsimulation()

# State-level
sim = Microsimulation(dataset='hf://policyengine/policyengine-us-data/states/NY.h5')

# Congressional district - SEE policyengine-district-analysis skill for full examples
sim = Microsimulation(dataset='hf://policyengine/policyengine-us-data/districts/NY-17.h5')
```

**For congressional district analysis** (representative's constituents, district-level impacts), use the `policyengine-district-analysis` skill which has complete examples.

## Key MicroSeries Methods

```python
income = sim.calc('household_net_income', period=2026, map_to='person')

income.mean()           # Weighted mean
income.sum()            # Weighted sum
income.median()         # Weighted median
(income > 50000).mean() # Weighted share meeting condition
```

## Finding Parameter Paths

```bash
grep -r "salt" policyengine_us/parameters/gov/irs/ --include="*.yaml"
```

**Parameter tree:** `gov.irs.deductions`, `gov.irs.credits`, `gov.states.{state}.tax`

**Patterns:** Filing status variants (SINGLE, JOINT, etc.), bracket syntax `[index]`, date format `'YYYY-MM-DD.YYYY-MM-DD'`