Essentially, they argue, because StumbleUpon users are so disproportionately active, any sample that misses some of them will vastly underrepresent their impact in driving traffic.
FORBES: Does StumbleUpon Really Drive More Traffic Than Facebook?
2.
Schools with few English learners may get no money at all, and federal help is heavily tied to census figures, which are believed to significantly underrepresent the number of students in need of services.
NPR: Instruction Of Students Learning English Bleak