Abstract
This is the third of four papers to be published in Research Ethics Review in 2009, that address methodological issues of relevance to research ethics committees. It focuses on three issues: the representativeness of study participants, the size of the study and data analysis. Differences between best practices in qualitative and quantitative research are highlighted. The paper argues that, while lack of representativeness may not be unethical, the ethical implications of unnecessary restrictions on eligibility should be considered by committees. Studies that are either too small or too big can pose problems. Research teams need to have the necessary competence to enable an appropriate analysis focused on the main objectives and interpreted in the context of the limitations of the study design.