The countries are all relatively close to the line, indicating that the model explains most of the variation in growth rates across countries. In fact, adding cognitive skills to a basic model that just includes initial income and years of schooling increases the share of cross-country variation in economic growth explained by the model from about one-quarter to about three-quarters. The quantity of schooling is statistically significantly related to economic growth in a specification that neglects educational quality, but the association between years of schooling and growth turns insignificant and is reduced to close to zero once cognitive skills are included in the model. In other words, added years of schooling do not affect growth unless they yield greater achievement.