Schools have previously been judged by how many pupils receive 5 good GCSE's or how many archive a Level 4 in their SAT's in Year 6. These measures are rather crude though. It doesn't take into account how well the children were performing when they entered the school. If the school selects on ability, then they should show a fantastic percentage pass rate for 5 GCSE's. That is where Progress 8 comes in. Without going into great detail here, it measures a school on the progress children have made since starting at the school.
This 2019 data is based on the school results from summer 2018 and released by gov.uk in December 2018