Research
We publish 102 tax calculators as open models, then check them against the law they claim to implement. These are the findings — figures, not impressions.
We audited 102 tax calculators. 29 were wrong.
We re-derived 102 net-salary and vehicle-tax models against primary sources. 29 of the 51 salary models needed a figure changed. Here is what was wrong, and what kind of wrong it was.
Three ways a calculator is wrong that its own tests can never catch
A green test suite proves a model computes what it says. It says nothing about whether what it says is right. Three failure modes from auditing 102 tax models, and what actually finds them.
Round numbers are the tell: spotting an invented rate table
Real statutory rate tables are ugly. When a calculator shows you four clean round numbers where the law has thirty bands, you are looking at something plausible rather than something true.