"The effects of characteristics of the studies, of selected covariates (tobacco) and of the gender of individuals included in the studies, were also investigate

"The effects of characteristics of the studies, of selected covariates (tobacco) and of the gender of individuals included in the studies, were also investigated as putative sources of heterogeneity of the estimates." http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2363992/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2363992/ Take a look at the papers before saying things like this.