To ensure the representativeness of the data, they interviewed 1000 people from diverse backgrounds.

representativeness 是由形容词 representative（有代表性的）加上名词后缀 -ness 构成的名词形式，表示"代表性"这个抽象概念。representative 本身由动词 represent（代表）加上后缀 -ative（具有...性质的）构成。