In May 2001, the quarterly Income and Expenditure Accounts adopted the Fisher index formula, chained quarterly, as the official measure of real expenditure-based Gross Domestic Product. There are two reasons for the adoption of this particular formula: it produces the most accurate measure of quarter to quarter growth in GDP and its components; and, the change brings the Canadian measure in line with the US quarterly Income and Product Accounts which also use the chain Fisher formula to measure real GDP