1. A method of determining a risk assessment value of an event sequence which is a partially ordered set indicating some events of an event group of M kinds of events in a time series, wherein M is a finite natural number, the method comprising: converting, with a processing device, the partially ordered set into an approximate totally ordered set; calculating an M-dimensional feature vector from the totally ordered set obtained by the conversion; and calculating a projection matrix for calculating the risk assessment value on the basis of the calculated M-dimensional feature vector.