Article source: ConstructConnect
Explanation of Quarterly and Annual GDP Percent Change Calculations
The mathematics employed to calculate ‘real’ gross domestic product (GDP) levels and rates of change are more convoluted than one might suppose. For a new quarter, GDP line items (e.g., consumption, investment, government spending and exports/imports), after removing the effects of inflation, are added up and adjusted for seasonality. They are also expressed as if they are annual results – i.e., the quarterly figures are ‘blown up’ to a corresponding annual level.
The ‘official’ GDP figure for any year is the average of the levels for the four quarters within that year and the year-to-year growth rate is the percentage change between annual averages.
The figure most often quoted by the press, however, is a quarter-to-quarter GDP growth rate annualized. Such a number compares latest-quarter GDP with previous-quarter GDP to derive a percentage change. Then that percentage change is compounded to the power of four (i.e., ‘annualized’) to account for four quarters in a year.
