Because 𝑙𝑥 is defined by a mortality table based on whole ages, it is necessary to obtain a value for 𝑙𝑥+𝑡 at a fractional age. A simple way of doing this is by assuming a uniform distribution of deaths (UDD) throughout the year of age 𝑥. This method is commonly used and is considered to be sufficiently accurate for all practical purposes.

This approximation assumes that 𝑙𝑥+𝑡 is a linear function of 𝑡 for integral 𝑥 and 0 ≤ 𝑡 ≤ 1.

This gives us mortality table values at fractional ages, which are needed in order to perform the actuarial calculations.