Pension tax relief calculations

Hi there!

I really enjoy OpenFisca, but I recently encountered an issue.

Here is what I did:

Set up one household with self-employed income of £30,000 (ID: 11800)
Set up one household with self-employed income of £30,000 and private pension contributions of £3000 (ID: 11799)
Set up one household with self-employed income of £60,000 (ID: 11817)
Set up one household with self-employed income of £60,000 and private pension contributions of £6000 (ID: 11816)

Here is what I expected to happen:

• In the case of the basic tax rate payers (11799/11800):
  • the income tax to be the same for both
  • the marginal tax rate for both to be 20%
• In the case of the higher rate tax payers (11816/11817):
  • the income tax to be reduced for 11816 by £1200 (20% of £6000)
  • the marginal tax rate for both to be 40%
• For both:
  • the net income should reflect the 'tax relief at source' of +20%
    • I guess this is more of a design point. Like what is 'net income' defined as? With these pension payments you are locking away the money until you retire, so maybe it should count towards some measure of 'wealth' or 'deferred income'? Because net income sounds like what you take-home, which is reduced if you are paying money to your pension.
    • Either way this tax relief at source is not accounted for.

Here is what actually happened:

• In the case of the basic tax rate payers (11799/11800):
  • 11799 had an income tax reduction of £600
  • The marginal tax rate for 11799 is marked as 0% (on the right hand side panel)
• In the case of the higher rate tax payers (11816/11817):
  • The income tax for 11816 is £2400 lower
  • The marginal tax rate for 11816 is marked as 0% (on the right hand side panel)

Here is data (or links to it) that can help you reproduce this issue:

https://www.gov.uk/tax-on-your-private-pension/pension-tax-relief

Context

I identify more as a:

• Commenter (I make data visualisations).
  (If I had to pick one of those, otherwise a hobbyist!)

Reproduce the four household cases using IDs 11800, 11799, 11817, and 11816, then compare income tax, marginal tax rate, and net income with the expectations in the issue. Use the linked GOV.UK pension tax-relief guidance to verify the intended treatment; done means the basic- and higher-rate cases produce the stated reductions, marginal rates, and relief-at-source behavior.