It cannot be determined how the failure to meet quality controls affected the data, but the failure greatly lowers the confidence one can place in any interpretation. Even if the criteria were met or did not exist, the data would be difficult to interpret because of the various differences among nations. When one considers that the students who were tested were mostly 18 years or older, the meaningfulness of determining comparability across countries based on enrollment figures for ages 12 to 17 is likewise called into question. It appears to be inaccurate, but attempts to track down the source have not been successful. The OECD figure for U.S. enrollment is contradicted in several places by other publications presenting the same statistic.
Medians are somewhat different.
The complete table from which Forgione's figure was taken is presented in Table 3 . American students were tested in May of their senior year and their average age was 18.1 years. In any case, an examination of the proportions of students in school in the grades tested is often much smaller than the enrollment figures used and further undermines confidence in the quality controls.