Sample size
Is the sample size justified?
Big enough to have high chance to detect a statistically significant effect?
Big enough to avoid type 1 error (more serious) and type 2 error?
Definitions of type 1 and type 2 errors?
Common problem: Research did not recruite enough subjects (small sample size)
Common consequence: Not enough subjects make a type 2 error more likely
Sampling technique
Has the sample population been selected in a clear and correct way?
Is the sampling technique clearly defined? (simple random sampling, strafied sampling, multistage sampling, purposive sampling, convenience sampling ...)
Is there a selection flow chart?
If not, students make the chart, using the info provided in the methodology
Are the inclusion and exclusion criteria well defined?