A biologist decided to change the textbook they used for the course ‘Introductory Biostatistics'. The average mark gained by the students in this course every year from 2004 to 2009 was very similar, but increased by 12% after introduction of the new text in 2010. The biologist said ‘The increase may have been due to the textbook, but you can never really be sure about that.' What did the biologist mean?