A three-year band is used in the Survey of Adult Skills to increase size and reliability of estimates, i.e. the group "adults 24” consists of the age groups from 23 to 25. The mix of countries contributing to the average in PISA and the Survey of Adult Skills differs, which may contribute to differences in countries’ average scores relative to the overall averages in either study. The learning environment is also closely related to teacher quality, as reported by principals (SNAE, 2013a).