The invention provides a method for selecting one or more genes from among a predetermined group of genes whose expression level is significantly different among a first group of individuals in a population in comparison with a second group within the population. The first group of individuals may be, a group of individuals suffering from an illness that responded to a predetermined treatment. The second group of individuals may be a group of individuals suffering from the same illness that did not respond to the treatment. For each of one or more selections of m individuals from the first subset and n individuals from the second subset, the statistical significance of each gene i in the up group of the genes, and in the down group of genes are determined, where the up group is a group of genes for which the expression level in the first group is greater than an expression level in the second group and the down group is a group of genes for which the expression level in the first group is less than an expression level in the second group. The genes in the up and the down groups are ranked according to the number of times each gene received the highest statistical significance among the one or more selections of individuals. A subset of genes in the up group having the highest rankings and a subset of genes in the fown group having the highest rankings are then selected.