The reading challenge records

At the end of the reading challenge, the organiser tried to compare the results of two classes. Class 9A had recorded the number of books each pupil finished, while Class 9B had recorded the number of pages each pupil read. Several of Class 9B's record sheets for the final week had been misplaced. Some pupils in Class 9A had included books they finished before the challenge began. In Class 9B, pages read at school had sometimes been entered again when pupils completed their records at home.

Describe four features of the records that make a fair comparison between the classes difficult. You may support your points with brief quotations.