The life expectancy of many types of lamps is defined as the number of working hours in which 50% of the lamps fail, i.e. the average life of the lamp. A production tolerance of as little as 1% results in a 25% difference in bulb life, so in general, some bulbs fail before the rated life expectancy, while others have a longer life. For LEDs, lamp life is defined as 50% of the light output of the lamp is reduced by 70% of the working time.

Some types of lamps are also sensitive to switching cycles. In rooms with frequent switches, such as bathrooms, the life expectancy of the light bulb is much shorter than that printed on the packaging box. Compact fluorescent lamps are particularly sensitive to switching cycles.