Upper and lower limits of predetermined characteristic values of products contained in a plurality of product lots are stored in accordance with the product standard for a target product. An average value of standard deviations in the characteristic values is calculated based on a control chart for the product lots. An average value of the characteristic values is calculated, and an upper limit and a lower limit of an average value of the characteristic values in a 95% confidence interval is calculated. A measurement standard deviation representing a variation in a measuring instrument with regard to the characteristic values is estimated. One of an upper limit and a lower limit of the average value of the characteristic values in the confidence interval is updated as an average value of the characteristic values. A standard deviation in the characteristic values of the product is estimated, and an upper defect rate and a lower defect rate are calculated, so that a yield rate is calculated.