Abstract: A system performs: acquiring historical data in which for each unit period, a high price and a low price of transaction price are related to a start time or an end time of a unit period; identifying the lowest price and the highest price of transaction price from the acquired historical data; acquiring a transaction price at predetermined time intervals; and based on the identified lowest price and highest price and the transaction price, calculating an index value indicating a trend in the transaction price. The system performs: at each time of acquiring the transaction price, based on the acquired transaction price and the identified lowest and highest prices, updating the lowest price or the highest price of transaction price; and based on the updated one of the lowest price and the highest price, the other one not updated, and the acquired transaction price, calculating the index value.