In research, many studies are non-randomized, so risk of bias.
Newcastle-Ottawa scale is one attempt to assess bias formally – judged on:
- the selection of the study groups;
- the comparability of the groups;
- the ascertainment of either the exposure or outcome of interest for case-control or cohort studies respectively.
So things like cohort not being representative; control group coming from different population; measurement being rather subjective; duration of follow up – all increase risk of bias.
Publication bias
Suspicious if small study with big effect!