Quite clearly, the average student in Ghana or Peru does
not gain the same amount of knowledge in any year of
schooling as the average student in Finland or Korea;
however, using measures of years of schooling assumes
that they are equivalent. In addition, using years of schooling
implicitly assumes that all skills and human capital
come from formal schooling. Yet, extensive evidence on
knowledge development and cognitive skills indicates that
a variety of factors outside of school – family, peers, and
others – have a direct and powerful influence. Ignoring
these nonschool factors introduces another element of
measurement error into the growth analyses.