A computer system for calculating a quantity from a set of input values. The computer system comprises a database that is configured to store a sequence of first input values xn wherein n=1, 2, . . . i, wherein for any value of n, xn lies between xn+1 and xn−1 in the sequence. Each value of x has associated with it a second input value yn, a conversion value pn that is calculated from yn according to a conversion algorithm, and an output value zn, wherein zn=zn−1+pn. The database is configured to store the plurality of output values zn.