2.4 Statistical analysis
Meta-analysis of this study included two aspects. In the primary
aspects, pooled estimates of the OR were estimated for the associations
of PD-L1 expression in clinical parameters, which were computed by
natural logarithm OR, and its standard errors (SEs). In the second
aspects, pooled estimates of HR of PD-L1 related to prognostic factors
were performed by a random-effects model, in which natural logarithm HR
and its SEs were calculated. The estimated HR of individual study was
preferred to entered to this analysis if they was reported in the case
of various adjusted factors. Several studies did not provided direct HR,
but its information were available for synthesizing the estimated HR,
and then methods described in Tierney et al.14 were
performed to impute the estimated HR. We assumed clinicopathological and
prognostic significance of PD-L1 expression was confounded by methods
for PD-L1 immunostaining such as antibody, definition of PD-L1
positivity, IHC cutoffs. Studies reporting more than one type of them
were separated as different data sets, correspondingly.
The heterogeneity across studies was investigated through Cochran’s Q
test (p < 0.1) and Higgins I2, of which
values 25%, 50% and 75% respectively indicated low, mild and high
heterogeneity15. In order to identify the sources of
heterogeneity, subgroup analyses were performed on the basis of grouping
by assumed confounding factors. Pooled estimates synthesized by at least
three studies were considered to be robust that no obvious fluctuation
happened when removing one study at a time. Publication bias was
determined by Begg’s and Egger’s tests16.