 The standard approach to this issue in economic analysis treats preference as based on value. Value is dealt with in a bifactorial model, in which the value of a future good is assumed to be equal to the product of two factors. One of these factors is a time-independent evaluation of the good in question, i.e. the value of obtaining it immediately. The other factor represents the subject’s pure time preferences. It is a function of the length of the delay, and is the same for all types of goods. The most common type of time preference function can be written