1. A computer system for calculating a quantity from a set of input values, the computer system comprising: a database that is configured to store a sequence of first input values x n wherein n=1, 2,. .. i, wherein for any value of n, x n lies between x n+1 and x n−1 in the sequence, and has associated with it: a second input value y n ; a conversion value p n that is calculated from y n according to a conversion algorithm; and an output value z n , wherein z n =z n−1 +p n ; the database being further configured to store the plurality of output values z n .