Critics argue that it is not possible to compare the prices of manufactured goods over time because they change rapidly. The price relationship of Prebisch–Singer does not take into account technological change. The important thing is not the price of the goods but the service provided by said goods. For example, in 1800 an American worker could buy a candle that provided one hour of light for six hours of work. But in 1997 an American worker could buy an hour of light provided by a light bulb with barely half a second of work. That is to say, the invention of Edison improved by other North Americans managed to reduce the price drastically. Another case that we can see are personal computers that provide the service of calculations per second. Since the 1970s computers doubled their capacity of calculations per second every two years for the same amount of constant dollars. The fall in price is so rapid, that it has been necessary to invent new words because of the immense growth in the capacity of computers. First they were measured in bytes, then, kilobytes, megabytes, terabytes, yottabytes, etc ... Today's harvesters harvest many more hectares per hour than they did half a century ago, but they also have a geo-satellite system, combined with a chip that allows to improve productivity; plus air-conditioned, hermetic cabinets, which prevent dust intake and improve the quality of life of the operator, as well as radio and DVD player to improve his comfort.