HMRC’s quality assurance arrangements involve testing 400 of its completed compliance cases each year. In 2021–22, 80 of these cases had errors in the compliance yield that HMRC had recorded. These errors included both overstated and understated figures, and occurred for a range of reasons including data input mistakes, incorrect calculations, or incorrect decisions on what should or should not be included. However, they were skewed towards overstating compliance yield. Out of £736 million of yield that was tested, the net effect of the errors was to overstate compliance yield by £59.7 million (8%).33