Table 3.

Bias matrix for prospective cohort studies.

Parameters of NOS scale	Kumar 2004	Wu 2010	Chen 2014	Sun 2014	Kitchlu 2015
Representatives of exposed cohort	L	L	L	L	L
Selection of non-exposed cohorts	L	L	L	L	L
Ascertainment of exposure	L	L	L	L	L
Outcome of interest not present	L	L	L	L	L
Comparability of cohort (age wise)	H	L	L	UC	L
Comparability of cohort (severity of illness wise)	H	L	L	UC	L
Assessment of outcome	L	L	L	L	L
Was follow-up long enough	L	L	L	L	L
Adequacy of follow up of cohorts	UC	L	L	L	L

L = low risk of bias, H = high risk of bias, UC = unclear risk of bias.