The harmonised index of consumer prices is a chain index in which the weights of the main groups are updated annually. For subgroups, weights are revised when they are no longer representative of current consumption patterns. As it is a chain index, products may be added or withdrawn from the basket of goods and services on an annual basis. The table below provides an overview of the weights in recent years. Since 2011 the weight of each group has been based on national accounts, whereas they were previously derived from the Household Budget Survey. Since 2012 the use of national accounts is required for the determination of HICP weights.