Illustration of pathway-level aggregation of gene expression data. (A) A gene expression data in which the effect of smoking on bronchial epithelium was investigated by comparing current smokers and never smokers (the Beane dataset in Table 1). To save space, expression levels of 76 selected genes are shown, which are the member genes of two major pathways mediating the effect of smoking. (B) Corresponding pathway expression profiles aggregated by the six compared methods. With this compact representation of the original gene-level data, it becomes easier to recognize that the two pathways are up-regulated by smoking.