Table 1. Quality assessment using CASP tool.

KEY: Y=YES, N=NO, ?=cannot tell

CASP, Critical Appraisal Skill Program; CI, confidence interval

S. no.	Questions	Ray et al. [15]	Lau et al. [16]	Alberts et al. [17]	Huang et al. [18]	Jansson et al. [19]
1	Did the study address a clearly focused issue?	?	Y	Y	Y	Y
2	Did the authors use an appropriate method to answer their question?	Y	?	Y	Y	Y
3	Were the cases recruited in an acceptable way?	?	Y	Y	Y	Y
4	Were the controls selected in an acceptable way?	Y	Y	Y	Y	Y
5	Was the exposure accurately measured to minimize bias?	N	N	Y	Y	?
6(a)	Aside from the experimental intervention, were the groups treated equally?	Y	Y	N	Y	Y
6(b)	Have the authors taken account of the potential confounding factors in the design and/or in their analysis?	N	Y	?	?	Y
7	How large was the treatment effect?	The study measures HR: 1.12; 95% CI: 1.05–1.21, P=0.01.	HR: 0.72; 95% CI: 0.66–0.79, P=0.01	The study predicted close HR values for outcome.  HR: 0.81; 95% CI: 0.73–0.91, P=0.01	HR: 0.79; 95% CI: 0.66–0.94, P=0.01	The study showed a significant  Standardized absolute risk: 1.73% (1.43% to 2.03%)
8	How precise was the estimate of the treatment effect?	p<0.0001. The results validate the study hypothesis.	P=0.05. The overall effect size showed no significance	The analysis had a linear relation (p=0.05)	Statistically significant association with p<0.001	Statistically significant association with p<0.001
9	Do you believe the results?	Y	Y	Y	Y	?
10	Can the results be applied to the local population?	Y	N	Y	N	N
11	Do the results of this study fit with other available evidence?	N	Y	?	Y	?
Score out of 11		7	9	9	10	8