Because of their poor efficiency, incandescent lamps contribute a large amount of heat to a building and have relatively high operating costs. Their short lamp life results in potentially high maintenance costs, since employees must replace them often. Lamp life refers to the time it takes half of the lamps in a given sample to fail. If a lamp is rated as having a lamp life of 1,000 hours, for example, 1,000 hours is the expected lifetime for this type of lamp, although a given lamp might burn for more or fewer than 1,000 hours