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

usefulness 是由形容词 useful（有用的）加上名词后缀 -ness 构成的名词形式，表示"有用性"或"实用性"。后缀 -ness 通常加在形容词后，将其转化为抽象名词，表示某种性质或状态。