the imputation of components for students missing data on one component was done on the basis of a regression on the other two variables with an additional random error component figure presents the percentage of students performing below proficiency level for participating countries including mexico as well as relative changes between and mexico is one of the four countries that showed statistically significant increases in the percentage of top performing students going from in to in t the other three countries were portugal with an increase of more than four percentage points to almost italy with nearly two percentage points to and greece with an increase of almost two percentage points to nearly