The lunchtime survey

Our class wanted to compare pupils' views of the new library. We planned to ask everyone the same four questions in a quiet room. At lunchtime, a football match began outside, and several answers could not be heard on the audio recorder. One group was asked a different final question because I read from an older sheet. Some pupils gave very brief replies because they wanted to catch the bus. Halfway through the survey, I began adding extra prompts that the earlier pupils had not received.

Describe four difficulties that made the survey responses harder to compare. You may use short quotations from the passage.