PolicyEngine Period Patterns

Essential patterns for handling different definition periods (YEAR, MONTH) in PolicyEngine.

Quick Reference

From	To	Method	Example
MONTH formula	YEAR variable	period.this_year	age = person("age", period.this_year)
YEAR formula	MONTH variable	period.first_month	person("monthly_rent", period.first_month)
Any	Year integer	period.start.year	year = period.start.year
Any	Month integer	period.start.month	month = period.start.month
Annual → Monthly	Divide by 12	/ MONTHS_IN_YEAR	monthly = annual / 12
Monthly → Annual	Multiply by 12	* MONTHS_IN_YEAR	annual = monthly * 12

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1. Definition Periods in PolicyEngine US

Available Periods

• YEAR: Annual values (most common - 2,883 variables)
• MONTH: Monthly values (395 variables)
• ETERNITY: Never changes (1 variable - structural relationships)

Note: QUARTER is NOT used in PolicyEngine US

Examples

from policyengine_us.model_api import *

class annual_income(Variable):
    definition_period = YEAR  # Annual amount

class monthly_benefit(Variable):
    definition_period = MONTH  # Monthly amount

class is_head(Variable):
    definition_period = ETERNITY  # Never changes

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2. The Golden Rule

When accessing a variable with a different definition period than your formula, you must specify the target period explicitly.

# ✅ CORRECT - MONTH formula accessing YEAR variable
def formula(person, period, parameters):
    age = person("age", period.this_year)  # Gets actual age

# ❌ WRONG - Would get age/12
def formula(person, period, parameters):
    age = person("age", period)  # BAD: gives age divided by 12!

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3. Common Patterns

Pattern 1: MONTH Formula Accessing YEAR Variable

Use Case: Monthly benefits need annual demographic data

class monthly_benefit_eligible(Variable):
    value_type = bool
    entity = Person
    definition_period = MONTH  # Monthly eligibility

    def formula(person, period, parameters):
        # Age is YEAR-defined, use period.this_year
        age = person("age", period.this_year)  # ✅ Gets full age

        # is_pregnant is MONTH-defined, just use period
        is_pregnant = person("is_pregnant", period)  # ✅ Same period

        return (age < 18) | is_pregnant

Pattern 2: Accessing Stock Variables (Assets)

Stock variables (point-in-time values like assets) are typically YEAR-defined

class tanf_countable_resources(Variable):
    value_type = float
    entity = SPMUnit
    definition_period = MONTH  # Monthly check

    def formula(spm_unit, period, parameters):
        # Assets are stocks (YEAR-defined)
        cash = spm_unit("cash_assets", period.this_year)  # ✅
        vehicles = spm_unit("vehicles_value", period.this_year)  # ✅

        p = parameters(period).gov.tanf.resources
        return cash + max_(0, vehicles - p.vehicle_exemption)

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4. Understanding Auto-Conversion: When to Use period vs period.this_year

The Key Question

When accessing a YEAR variable from a MONTH formula, should the value be divided by 12?

• If YES → Use period (let auto-conversion happen)
• If NO → Use period.this_year (prevent auto-conversion)

When Auto-Conversion Makes Sense (Use period)

Flow variables where you want the monthly portion:

class monthly_benefit(Variable):
    definition_period = MONTH

    def formula(person, period, parameters):
        # ✅ Use period - want $2,000/month from $24,000/year
        monthly_income = person("employment_income", period)

        # Compare to monthly threshold
        p = parameters(period).gov.program
        return monthly_income < p.monthly_threshold

Why: If annual income is $24,000, you want $2,000/month for monthly eligibility checks.

When Auto-Conversion Breaks Things (Use period.this_year)

Stock variables and counts where division by 12 is nonsensical:

1. Age

# ❌ WRONG - gives age/12
age = person("age", period)  # 30 years → 2.5 "monthly age" ???

# ✅ CORRECT - gives actual age
age = person("age", period.this_year)  # 30 years

2. Assets/Resources (Stocks)

# ❌ WRONG - gives assets/12
assets = spm_unit("spm_unit_assets", period)  # $12,000 → $1,000 ???

# ✅ CORRECT - gives point-in-time value
assets = spm_unit("spm_unit_assets", period.this_year)  # $12,000

3. Counts (Household Size, Number of Children)

# ❌ WRONG - gives count/12
size = spm_unit("household_size", period)  # 4 people → 0.33 people ???

# ✅ CORRECT - gives actual count
size = spm_unit("household_size", period.this_year)  # 4 people

4. Boolean/Enum Variables

# ❌ WRONG - weird fractional conversion
status = person("is_disabled", period)

# ✅ CORRECT - actual status
status = person("is_disabled", period.this_year)

Decision Tree

Accessing YEAR variable from MONTH formula?
│
├─ Is it an INCOME or FLOW variable?
│  └─ YES → Use period (auto-convert to monthly) ✅
│           Example: employment_income, self_employment_income
│
└─ Is it AGE, ASSET, COUNT, or BOOLEAN?
   └─ YES → Use period.this_year (prevent conversion) ✅
            Examples: age, assets, household_size, is_disabled

Complete Example

class monthly_tanf_eligible(Variable):
    value_type = bool
    entity = Person
    definition_period = MONTH

    def formula(person, period, parameters):
        # Age: Use period.this_year (don't want age/12)
        age = person("age", period.this_year)  # ✅

        # Assets: Use period.this_year (don't want assets/12)
        assets = person("assets", period.this_year)  # ✅

        # Income: Use period (DO want monthly income from annual)
        monthly_income = person("employment_income", period)  # ✅

        p = parameters(period).gov.tanf.eligibility

        age_eligible = (age >= 18) & (age <= 64)
        asset_eligible = assets <= p.asset_limit
        income_eligible = monthly_income <= p.monthly_income_limit

        return age_eligible & asset_eligible & income_eligible

Pattern 3: Converting Annual to Monthly

class monthly_income_limit(Variable):
    definition_period = MONTH

    def formula(household, period, parameters):
        # Get annual parameter
        annual_limit = parameters(period).gov.program.annual_limit

        # Convert to monthly
        monthly_limit = annual_limit / MONTHS_IN_YEAR  # ✅

        return monthly_limit

Pattern 4: Getting Period Components

class federal_poverty_guideline(Variable):
    definition_period = MONTH

    def formula(entity, period, parameters):
        # Get year and month as integers
        year = period.start.year  # e.g., 2024
        month = period.start.month  # e.g., 1-12

        # FPG updates October 1st
        if month >= 10:
            instant_str = f"{year}-10-01"
        else:
            instant_str = f"{year - 1}-10-01"

        # Access parameters at specific date
        p_fpg = parameters(instant_str).gov.hhs.fpg
        return p_fpg.first_person / MONTHS_IN_YEAR

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5. Parameter Access

Standard Access

def formula(entity, period, parameters):
    # Parameters use current period
    p = parameters(period).gov.program.benefit
    return p.amount

Specific Date Access

def formula(entity, period, parameters):
    # Access parameters at specific instant
    p = parameters("2024-10-01").gov.hhs.fpg
    return p.amount

Important: Never use parameters(period.this_year) - parameters always use the formula's period

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6. Testing with Different Periods

Critical Testing Rules

For MONTH period tests (period: 2025-01):

• Input YEAR variables as annual amounts
• Output YEAR variables show monthly values (÷12)

Test Examples

Example 1: Basic MONTH Test

- name: Monthly income test
  period: 2025-01  # MONTH period
  input:
    people:
      person1:
        employment_income: 12_000  # Input: Annual
  output:
    employment_income: 1_000  # Output: Monthly (12_000/12)

Example 2: Mixed Variables

- name: Eligibility with age and income
  period: 2024-01  # MONTH period
  input:
    age: 30  # Age doesn't convert
    employment_income: 24_000  # Annual input
  output:
    age: 30  # Age stays same
    employment_income: 2_000  # Monthly output
    monthly_eligible: true

Example 3: YEAR Period Test

- name: Annual calculation
  period: 2024  # YEAR period
  input:
    employment_income: 18_000  # Annual
  output:
    employment_income: 18_000  # Annual output
    annual_tax: 2_000

Testing Best Practices

1. Always specify period explicitly
2. Input YEAR variables as annual amounts
3. Expect monthly output for YEAR variables in MONTH tests
4. Use underscore separators: 12_000 not 12000
5. Add calculation comments in integration tests

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7. Common Mistakes and Solutions

❌ Mistake 1: Not Using period.this_year

# WRONG - From MONTH formula
def formula(person, period, parameters):
    age = person("age", period)  # Gets age/12!

# CORRECT
def formula(person, period, parameters):
    age = person("age", period.this_year)  # Gets actual age

❌ Mistake 2: Redundant period.this_year / MONTHS_IN_YEAR for Flow Variables

# WRONG - Double-converting; period already auto-divides by 12
fpg = spm_unit("spm_unit_fpg", period.this_year) / MONTHS_IN_YEAR

# CORRECT - Core auto-converts annual → monthly when you use period
fpg = spm_unit("spm_unit_fpg", period)

This applies to all YEAR-defined flow variables (income, FPG, dollar amounts). Using period from a MONTH formula auto-divides by 12. Using period.this_year / MONTHS_IN_YEAR is equivalent but redundant — just use period.

❌ Mistake 3: Mixing annual and monthly

# WRONG - Comparing different units
monthly_income = person("monthly_income", period)
annual_limit = parameters(period).gov.limit
if monthly_income < annual_limit:  # BAD comparison

# CORRECT - Convert to same units
monthly_income = person("monthly_income", period)
annual_limit = parameters(period).gov.limit
monthly_limit = annual_limit / MONTHS_IN_YEAR
if monthly_income < monthly_limit:  # Good comparison

❌ Mistake 4: Wrong test expectations

# WRONG - Expecting annual in MONTH test
period: 2024-01
input:
  employment_income: 12_000
output:
  employment_income: 12_000  # Wrong!

# CORRECT
period: 2024-01
input:
  employment_income: 12_000  # Annual input
output:
  employment_income: 1_000  # Monthly output

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8. Real-World Example

class tanf_income_eligible(Variable):
    value_type = bool
    entity = SPMUnit
    definition_period = MONTH  # Monthly eligibility

    def formula(spm_unit, period, parameters):
        # YEAR variables need period.this_year
        household_size = spm_unit("spm_unit_size", period.this_year)
        state = spm_unit.household("state_code", period.this_year)

        # MONTH variables use period
        gross_income = spm_unit("tanf_gross_income", period)

        # Parameters use period
        p = parameters(period).gov.states[state].tanf

        # Convert annual limit to monthly
        annual_limit = p.income_limit[household_size]
        monthly_limit = annual_limit / MONTHS_IN_YEAR

        return gross_income <= monthly_limit