Normal spoilage rates are computed by dividing the units of normal spoilage by total good units completed, not total actual units started in production. At Mendoza Plastics, the normal spoilage rate is therefore computed as 400 ÷ 20,000 =2%. There is a tradeoff between the speed of production and the normal spoilage rate. Managers make a conscious decision about how many units to produce per hour with the understanding that, at the chosen rate, a certain level of spoilage is unavoidable.