<p>Events such as cognition and judgement for sensory stimulus have attracted attention in the fields of psychology, medicine, and engineering to investigate the process of information processing in brain. Questionnaire is often used as a method to evaluate the process of information processing in brain. In order to deal with human state which changes from time to time, method for evaluating events using biosignal is required. We used Auditory Event Related Potentials (ERPs) to evaluate events. ERPs contains P300 which appears by cognition and judgement for sensory stimulus. In this paper, we propose evaluation method for cognition and judgement using ERPs with P300. We used modified Duffing oscillator to investigate whether the parameters of mathematical model could be used in analysis. It was applied to EEG data which is obtained by ERPs measurement experiment. Interval estimation was used for parameters analysis. As a result, parameter A decreased and parameter B increased when ERPs appeared, and parameter C decreased after ERPs appeared. It is suggested that interval estimation can be a method which can evaluate events appearing in ERPs because it can investigate the change of model parameters due to cognition and judgement.</p>