They came back yesterday and I have interviewed them to find out what they think about the usefulness of the program.

usefulness 是由形容词 useful（有用的）加上名词后缀 -ness 构成的名词，表示“有用性、实用性”。-ness 是一个常见的名词后缀，用于将形容词转化为抽象名词。