1. A non-transitory computer readable recording medium storing therein a group forming program that causes a computer to execute a process comprising: extracting a plurality of keywords from public information of a first group of publishers including a publisher for which an attribute value regarding a first attribute is calculated as corresponding to a predetermined attribute value, the first attribute being an attribute with respect to each publisher publishing information, the attribute value of the first attribute varying with respect to passage of time; selecting, from the plurality of keywords, correlated keywords based on a correlation between the predetermined attribute value and each of the plurality of keywords; determining a number of the correlated keywords based on a difference between a distribution of the correlated keywords in the public information of the first group of publishers and another distribution of the correlated keywords in public information extracted by using the correlated keywords; forming a second publisher group with respect to new public information by forming a publisher group of the new public information containing the determined number of the correlated keywords; and forming a new publisher group corresponding to the first publisher group by using one or more publishers whose attribute value regarding estimated the first attribute is calculated as corresponding to the predetermined attribute value from among publishers contained in the second publisher group.