The codes denoting level of education attained in the United Kingdom in the variable EDUCUK refer to studies taken in the last two years of secondary school education, usually at ages 16 - 18. Additionally, for education level, the two samples (1991 and 2001) group countries that are part of the UK differently. In 1991, education level was collected for all countries in Great Britain under one variable. In 2001, this variable was split into two; one for England, Wales, and Northern Ireland who use a unified grading system and a second for Scotland who uses its own system. The 1991 codes (Levels A, B, and C) are part of the General Certificate of Education (GCE) system in which students study for A-Level (Advanced level) terminal exams; Level A corresponds with a passing grade of 80%, Level B corresponds with a passing grade of 70%, and Level C corresponds with a passing grade of 60% (for more information, see the original 1991 questionnaire). The 2001 codes for England, Wales, and Northern Ireland are based on the National Vocational Qualifications (NVQ), competence-based qualifications measuring the application of knowledge and skill-level of an individual with 1 being the lowest and 5 being the highest. Levels 4 and 5 are technically different but were grouped together in the original survey (for more information, see the original 2001 questionnaire). Scotland 2001 codes (Group 1, Group 2, etc.) can be roughly correlated with 2001 codes for England, Wales, and Northern Ireland (Level 1, Level 2, etc.) to indicate academic achievement.