1) The groups were randomised but were they similar at baseline? That is, we do not know whether all of the clever, verbose writers are in the group with the simple slides.

Fix: Participants write several baseline essays after randomisation using traditional and spare slides as source material before doing the test presented.

2) Did the participants understand the purpose of the research? I think it is common belief that 'less is more' for powerpoint presentations and participants may write better essays because they know they are in the 'sparse slides' group.

Fix: Blind participants to the research purpose (may require deception)

3) Do we really believe that number of themes mentioned is a good measure of an essay? Were they instructed to write about as many themes as possible? Perhaps the traditional slide group had significantly better understanding of the themes and were therefore able to write more details but cover less

Fix: Design a better way of measuring information retention.

That said, I'm all for sparse slides but I think we also need better research to prove its effectiveness.