Statistical analysis
The meta-mean with 95% confidence interval (CI) was calculated based on
the mean and SD of spirometry parameters. If a study only reported the
median, range and/or inter-quartile range (IQR); mean and SD were
estimated, according to Hozo et al (12). The Cochran Q statistic and
inconsistency index (I2) were used to assess the
heterogeneity among studies. If I2 was more than 50%,
and p value was lesser than 0.05, heterogeneity was considered
significant. The random effect model was used in significant
heterogeneity, whereas the fixed effect model was applied for
non-significant heterogeneity. To assess the stability of the results,
sequential omitting of individual studies in the meta-analysis was
performed using sensitivity analysis. Subgroups were analyzed based on
disease severity. Probable confounders were verified using meta
regression. The standardized mean difference (SMD) was calculated in
studies, which measured spirometry parameters twice. Publication bias
was assessed using Egger’s linear regression test. Agreement between
authors in data selection and extraction was assessed using Cohen’s
kappa statistic. Statistical analysis was performed using Comprehensive
Meta-Analysis (CMA) computer program (Biostat, Englewood, NJ). A p value
less than 0.05 was considered as statistically significant.