Because participating schools were offered a school-level report, which is of value to schools, some schools which were not in the selected sample opted into the survey.
In addition, schools which wanted to have a larger sample size and a wider range of year groups for their report could also opt in additional classes or year groups.
These cases were included in the data for school-level reports but were not included in the national data for publication or in the archived data, unless they were reserve sample schools which opted in.
Pupils from opt-in schools or classes not included in the national data were not given a weight in the dataset. Schools that opted in could complete the survey with any year groups and did not need to use latest class or teacher name in the alphabet for class selection.