A measurement system and a measurement method, which can obtain a measurement value close to a true value considering an overlay measurement error according to a higher order regression analysis model, is disclosed. The measurement system and the measurement method provide a technique for determining optimal positions of shots to be measured using an optimal experimental design. When the regression analysis model and the number of shots to be measured are determined in advance, a method is used for determining an optimal number of shots to be measured according to the regression analysis model and process dispersion using a confidence interval estimating method.