The tower in Pisa leans over in a predictable manner due to a three-meter foundation set in weak, unstable subsoil, according to authorities. The PISA tests, aka the Program for International Student Assessment, also produce predictable results across 72 countries with 500,000 tested students representing 28 million 15-year-olds.
PISA is not an achievement test. It assesses whether students can problem solve in various subject areas, including reading, math, science and financial literacy. Results from 2015 were reported December 6.