Example of the identification of the Oi in the meta-analysis method. Each of the circles represents a hypothetical dataset (S1 to S4). The numerals are the number of genes differentially expressed in the datasets represented by that area of overlap of the circles. The value Oi is defined as the number of genes differentially expressed in exactly i number of datasets. In the example, O1 is 85 since that is the number of genes differentially expressed in 1 study, whereas O4 is 1 since only one gene is present in the area overlapping all 4 studies.
Daves et al. BMC Medical Genomics 2011 4:56 doi:10.1186/1755-8794-4-56