Defining such a constant-quality unit of output, of course, is the central conceptual difficulty in price measurement. Such a definition may be clear for unalloyed aluminum ingot of 99.7 percent or greater purity in wide use. Consequently, its price can be compared over time with a degree of precision adequate for virtually all producers and consumers of aluminum ingot.
Similarly, the prices of a ton of cold rolled steel sheet, or of a linear yard of cotton broad woven fabric, can be reasonably compared over a period of years. But when the characteristics of products and services are changing rapidly, defining the unit of output, and thereby adjusting an item’s price for improvements in quality, can be exceptionally difficult.