A popular way to criticise any research study is to suggest that the sample was limited – but, in order to make this an effective criticism, you need to explain why this is an issue. For example, if the sample was all men then you need to explain why that would make a difference to performance or if the sample consisted solely of university students, why would that matter? In fact a large number of studies do use university students (easy to come by for university researchers). A recent study has given you support in using this as a criticism. Jonathan DeRight and Randall Jorgensen (2014) tested the effort made by students on a range of computerised tests which included tests that specifically assessed the amount of effort each participant was making (for example, seeing if participants performed worse than chance on a set of multiple choice questions). In one session they found that 1 in 4 students displayed low effort.
Therefore, a useful criticism of any study that used student participants is that research has shown that students are probably not putting much thought or effort into their performance in the study and thus the results may lack validity – and cite DeRight and Jorgensen.