In the purest system of classification, there are analog computers and digital computers. In an analog computer, data is represented by a continuous spectrum of voltages. They are prone to corruption from noise. Operations are performed in a parallel, the opposite of digital computers in which operations are performed sequentially using binary data. Although they can perform complex computer, the cost of the hardware can be prohibitive and they do not have the capacity to store large quantities of data. Analog computers are all but obsolete which is why some computer scientists no longer include them in their classification.