A method of selecting a factor for estimating characteristics of viewers who view a same content is disclosed. This method includes: storing a multi-layered hierarchal list of attribute words expressing viewer attributes, each of which is a candidate of the factor; extracting the attribute words from the same content; generating a first vector indicative of frequencies with which the attribute words occur in the same content; for a successively-selected one of the attribute words, successively generating a second vector indicative of frequencies with which the attribute words occur in ones of sets of learned-text information which contain the successively-selected attribute word; calculating a similarity score between the vectors on a per-attribute-word basis; and selecting one of the attribute words as the factor, which is associated with subordinate attribute words on a lower layer, based on a dispersion level of the similarity scores calculated for the subordinate attribute words.