Each year the National Reference Test (NRT) reports the proportions of students achieving at or above three key GCSE grades in English and mathematics. The NRT is therefore uniquely placed as a Key Stage 4 assessment which compares performance directly at ability level, to contribute to the evidence on the impact of disruption to education due to the COVID-19 pandemic. The main NRT outcomes, reported in the 2021 National Reference Test Results Digest (ED627839), show overall performance on the NRT in English and mathematics over time indicating how the subjects may have been impacted as a whole. This report describes whether the gaps in performance between students and schools, based on a range of demographic and attainment variables, changed significantly between 2020 and 2021. Two key themes were explored as part of this research: socio-economic factors and prior performance. Additionally, the analysis included a range of other demographic factors at both student- and school level that could potentially play a role in explaining how the impact of disruption due to the pandemic may vary across the population.