This memoryless property will not strictly be satisfied in all practical problems. For example, suppose that X is the length of time for which a light bulb will burn in the before it fails. The length of time for which the bulb can be expected to continue to burn in the future will depend on the length of time for which it has been burning in the past. Nevertheless, the exponential distribution has been used effectively as an approximate distribution for such variables as the lengths of the lives of various products