Potential Bug in Calculation of Wohngeldeinkommen

[michaelhebsaker]

Bug description

I think this code snippet does not reflect the de facto and de jure calculation of the Wohngeldeinkommen,

Version 0.4 code is:

vorläufiges_eink = (1 - wohngeld_abzüge_tu) * (
        wohngeld_brutto_eink_tu + wohngeld_sonstiges_eink_tu - wohngeld_eink_abzüge_tu
    )

Current code bakes in the wrong assumption

    vorläufiges_eink = (1 - wohngeld_abzüge_m_tu) * (
        wohngeld_eink_vor_abzug_m_tu - wohngeld_eink_abzüge_m_tu
    )

Expected behavior

Following:
https://www.bmi.bund.de/SharedDocs/downloads/DE/publikationen/themen/bauen/wohnen/wohngeld-flyer.pdf?__blob=publicationFile&v=9
Example 2, page 13

and:
https://www.wohngeld.org/wohngeldgesetz-wogg/paragraph14.html
see 14.21.20ff

I infer that wohngeld_eink_abzüge_tu as well as payments like Unterhaltsvorschuss (as I understand included in wohngeld_sonstiges_eink_tu) should not be multiplied with wohngeld_abzüge_tu.

This is escpecially relevant for single households with children.
Multiplying wohngeld_abzüge_tu with Unterhaltsvorschuss results in a Wohngeldeinkommen, which is - from my reading of the law -
too high.

Multiplying wohngeld_abzüge_tu with wohngeld_eink_abzüge_tu might lead to a too low wohngeldeinkommen.

[hmgaudecker]

This is great, thanks! Sounds plausible.

We are just in the middle of a sprint so that the released version of GETTSIM will be up-to-date at the workshop next week. So don't expect any action until then. Thinking of it, this might actually be a nice case to discuss / go through.

[michaelhebsaker]

Alright, thx for the warm words. Looking forward for the workshop and the discussions.

[ritter9]

To add to Friday's workshop discussion, in particular with reference to household vs Bedarfsgemeinschaft questions:

The reason Unterhaltsvorschuss does not get multiplied with wohngeld_abzüge_tu seems to be that these payments are added to the child's income rather than the adults and that wohngeld_abzüge_tu applies to each household member's income individually before they are all summed up to the household's yearly income.

See § 14 WoGG which defines the yearly income of one member of the household to be

die Summe der positiven Einkünfte im Sinne des § 2 Abs. 1 und 2 des Einkommensteuergesetzes zuzüglich der Einnahmen nach Absatz 2 abzüglich der Abzugsbeträge für Steuern und Sozialversicherungsbeiträge (§ 16).

And the households total income is defined by §13 WoGG as

Summe der Jahreseinkommen (§ 14) der zu berücksichtigenden Haushaltsmitglieder abzüglich der Freibeträge (die §§ 17 und 17a) und der Abzugsbeträge für Unterhaltsleistungen (§ 18).

As children do not pay taxes or health insurance fees, that income is not subjected to wohngeld_abzüge_tu and the Unterhaltsvorschuss gets added in full to the household's yearly income.

Legally this also means that wohngeld income should technically first be calculated for each household member individually before being summed up and then subject to the Freibeträge and Unterhaltsleistungen. I don't know if this is strictly necessary but it might be useful to keep this legal framework in mind when implementing the corrections to what Michael pointed out.

[cpz-research]

I think the correct treatment of Unterhaltsvorschuss is as follows:

@LauraGergeleit, would be great if you can double-check, implement it that way and, if possible, at test cases.

def wohngeld_eink_vor_freib_m(  # noqa: PLR0913
    eink_selbst: float,
    eink_abhängig_beschäftigt: float,
    kapitaleink_brutto: float,
    eink_vermietung: float,
    arbeitsl_geld_m: float,
    sonstig_eink_m: float,
    eink_rente_zu_verst_m: float,
    elterngeld_m: float,
    wohngeld_abzüge_st_sozialv_m: float,
) -> float:
    """Sum gross incomes relevant for housing benefit calculation on individual level
    and deducting individual housing benefit subtractions.

    Parameters
    ----------
    eink_selbst
        See :func:`_eink_selbst`.
    eink_abhängig_beschäftigt
        See :func:`eink_abhängig_beschäftigt`.
    kapitaleink_brutto
        See :func:`kapitaleink_brutto`.
    eink_vermietung
        See :func:`eink_vermietung`.
    arbeitsl_geld_m
        See :func:`arbeitsl_geld_m`.
    sonstig_eink_m
        See :func:`sonstig_eink_m`.
    eink_rente_zu_verst_m
        See :func:`eink_rente_zu_verst_m`.
    elterngeld_m
        See :func:`elterngeld_m`.

    Returns
    -------

    """
    einkommen = (
        eink_selbst + eink_abhängig_beschäftigt + kapitaleink_brutto + eink_vermietung
    ) / 12

    transfers = arbeitsl_geld_m + eink_rente_zu_verst_m + elterngeld_m

    eink_ind = einkommen + transfers + sonstig_eink_m
    out = (1 - wohngeld_abzüge_st_sozialv_m) * eink_ind

    return out
    

def wohngeld_eink_m_hh(
    anz_personen_hh: int,
    wohngeld_eink_freib_m_hh: float,
    wohngeld_eink_vor_freib_m_hh: float,
    unterhaltsvors_m_hh: float,
    wohngeld_params: dict,
) -> float:
    """Calculate final income relevant for calculation of housing benefit on household
    level.

    Parameters
    ----------
    anz_personen_hh
        See :func:`anz_personen_hh`.
    wohngeld_eink_freib_m_hh
        See :func:`wohngeld_eink_freib_m_hh`.
    wohngeld_eink_vor_freib_m_hh
        See :func:`wohngeld_eink_vor_freib_m_hh`.
    wohngeld_params
        See params documentation :ref:`wohngeld_params <wohngeld_params>`.

    Returns
    -------

    """
    wohngeld_eink_nach_abzug_m_hh = (
        wohngeld_eink_vor_freib_m_hh + unterhaltsvors_m_hh - wohngeld_eink_freib_m_hh
    )
    unteres_eink = wohngeld_params["min_eink"][
        min(anz_personen_hh, max(wohngeld_params["min_eink"]))
    ]

    out = max(wohngeld_eink_nach_abzug_m_hh, unteres_eink)
    return float(out)    
    
    

[LauraGergeleit]

@LauraGergeleit, would be great if you can double-check, implement it that way and, if possible, at test cases.

I double-checked your suggested implementation and when considering the previous discussion in this PR, the current implementation on the main branch appears correct to me, as we interpret the amount of unterhaltsvors_m as child's income. That way, wohngeld_abzüge_st_sozialv_m would be equal to 0 and the full amount of unterhaltsvors_m would be considered income, as it appears to be the correct way but let me know if I am missing something..