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Incorrect lagged income quintiles after benefit-unit changes

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
java
Domain
backend, databases

Research direction

Start with Person.getYHhQuintilesMonthC5L1() in src/main/java/simpaths/model/Person.java, the Q2–Q5 dummy getters, and new benefit unit initialization in src/main/java/simpaths/model/BenefitUnit.java. Trace how a person's benefit unit changes and decide how individual lagged quintile history should be retained, including how missing history is handled. Add focused checks for unchanged membership, a new unit at age 18, and partnership formation between people with different quintiles.

Written by the indexing model from the issue text.

Description

Coding problem

A person's lagged income quintile should describe the benefit unit they belonged to in the previous period. Instead, Person.getYHhQuintilesMonthC5L1() reads the lag stored on their current benefit unit:

return benefitUnit.getYHhQuintilesMonthC5L1();

When someone forms a new benefit unit, for example on turning 18 or forming a partnership, that unit may have no previous-period quintile. The Q2-Q5 dummy getters then all return zero, implicitly applying the Q1 reference-category coding.

The new-unit initialisation supplies Q3 for a missing current quintile, but does not restore the person's previous quintile.

Verification

In a clean build of develop at 11bec2bd9 (23 October), an isolated getter test returned:

  • Original benefit unit with lagged Q5: Q5.
  • Membership changed to a new unit without history: null, with all Q2-Q5 dummy values 0.
  • Destination unit given lagged Q2: Q2, rather than the person's original Q5.

The same behaviour occurs in the saved income-validation JAR. These checks establish the getter problem; they do not measure its population frequency or aggregate effect on predictions.

Proposed coding repair

The approach discussed was to preserve each person's BU income quintile at the individual level before membership changes, and use that stored previous-period value for the lagged regressor. Two people forming a couple could then correctly retain different lagged quintiles. Retaining the previous BU link and its historical value is an alternative.

A focused test would check unchanged membership, a new BU at age 18, and partnership formation between people from different quintiles. Missing history would also need explicit handling rather than silently becoming the reference category. This repair preserves the meaning of the existing regressor; it does not itself require replacing it with parental income.

Separate education-specification question

Even with the coding repair, a student may subsequently have low own-BU income while still living with well-off parents. Whether E1a should instead use parental income, and how E1b should distinguish own from parental resources, is a separate specification decision requiring consistent estimation and Java implementation.

Could we confirm the preferred implementation and who will take it forward?

Dominant language
Java
Stars
26
Forks
38
Avg merge
11h 45m
Merged PRs (30d)
8

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