1. A computer-readable medium encoded with a working skill estimating program for estimating working skills of an agent, the program, when executed by a computer, causing the computer to function as: a large category classifying unit to classify response records collected during a predetermined period into large categories according to predetermined classification rules based on topics of inquiries and responses recorded in the response records; a small category generating unit to generate small categories by further classifying the response records belonging to each of the large categories according to a predetermined definition of similarity between individual response records; a disturbance distinguishing unit to classify the response records belonging to each of the generated small categories into common response records that are produced constantly over time or disturbances whose frequency exhibits a significant time dependency; and a skill estimating unit to select one of the large categories, extract the response records belonging to the selected large category while removing therefrom the response records of the small categories that the disturbance distinguishing unit has classified as disturbances, and estimate working skills of the agent in the selected large category, based on at least how many response records are extracted.