Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime.:5 There are several measures of durability in use, including years of life, hours of use, and number of operational cycles. In economics, goods with a long usable life are referred to as durable goods. Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime.:5 There are several measures of durability in use, including years of life, hours of use, and number of operational cycles. In economics, goods with a long usable life are referred to as durable goods. Product durability is predicated by good reparability and regenerability in conjunction with maintenance. Every durable product must be capable of adapting to technical, technological and design developments. This must be accompanied by a willingness on the part of consumers to forego having the 'very latest' version of a product. Durability as a characteristic relating to the quality of goods that can be demanded by consumers was not clearly established in the United Kingdom until an amendment of the Sale of Goods Act 1979 relating to the quality standards for supplied goods in 1994. The lifespan of household goods is a significant factor in sustainable consumption. Longer product life spans can contribute to eco-efficiency and sufficiency, thus slowing consumption in order to progress towards a sustainable level of consumption. Cooper (2005) proposed a model to demonstrate the crucial role of product life spans to sustainable production and consumption.