PISA is an influential international study of the achievement of 15-year-olds. It has a high profile across the devolved nations of the UK, with the results having a substantial impact upon education policy. Yet many of the technical details underpinning PISA remain poorly understood--particularly amongst non-specialists--including important nuances surrounding the representivity of the data. This paper provides new evidence on this issue, based upon a case study of PISA 2018. I illustrate how there are many anomalies with the data, with the combination of nonresponse, exclusions from the test and technical details surrounding eligibility criteria leading to total nonparticipation rates of around 40% (amongst the highest anywhere in the world). It is then shown how this leads to substantial uncertainty surrounding the PISA results, with clear evidence of bias in the sample for certain parts of the UK. I conclude by discussing how more transparent reporting of the technical details underpinning PISA is needed, at both a national and international level.