HMRC does not know to what extent the errors identified are representative of compliance yield as a whole being overstated. HMRC also cannot estimate how many taxpayers it is overcharging following compliance enquiries, or by how much. HMRC explained that it designed its sampling process to examine quality in all areas of its casework, rather than to provide assurance on the overall estimate. Its approach means it cannot accurately extrapolate the errors or overcharges it finds to produce an overall estimate of their impact. HMRC told us it expects compliance yield to be materially correct but it cannot provide assurance that this is the case. Errors should be rare, and finding a lot of them in a sample may indicate a bigger problem that merits further testing. HMRC told us that it is reviewing how it can update its testing approach so that it can better estimate the extent of errors it makes, based on suggestions from the National Audit Office.37 Stopping the tax gap from growing