Accounting for Seasonality in GDP

BEA’s estimates of GDP are seasonally adjusted to remove fluctuations that normally occur at about the same time and the same magnitude each year.  Seasonal adjustment ensures that the remaining movements in GDP, or any other economic series, better reflect true patterns in economic activity.  Examples of factors that may influence seasonal patterns include weather, holidays, and production schedules. (See “Why and how are seasonal adjustments made?“)

Much of the data used by BEA to estimate GDP are seasonally adjusted by the source data agencies. For example, BEA uses seasonally-adjusted inventory and retail sales data from the U.S. Census Bureau and seasonally-adjusted consumer price indexes from the U.S. Bureau of Labor Statistics. BEA does seasonally adjust some data itself, such as Treasury data used to measure federal government spending. There are also instances where BEA cannot apply seasonal adjustment statistical techniques to its source data because the time series is too short to adequately capture seasonal trends.

BEA and its source data agencies regularly review and update their seasonal adjustment procedures to account for changes in seasonal patterns that emerge over time. Despite regular reviews and updates, changes in seasonal patterns can sometimes lead to ‘residual seasonality’—that is, the manifestation of seasonal patterns in data that have already been seasonally adjusted. There are several reasons that residual seasonality might arise:

  • After the detailed, individual components of GDP are seasonally adjusted, BEA aggregates the seasonally adjusted components to obtain total GDP. In some cases, seasonal patterns may emerge in the aggregate estimates that were not apparent in the individual components.
  • In some cases, the source data may be seasonally adjusted at monthly frequency, but in aggregating to quarterly frequency, seasonal patterns may emerge that were not apparent in the monthly data. To maintain consistency with the source data, BEA does not introduce different seasonal adjustments from those used in the monthly source data.
  • In some cases, the current-dollar values and prices may be independently seasonally adjusted, then the values are deflated (divided by the price) to obtain estimates of real expenditures. Seasonal patterns sometimes emerge in the deflated estimates that were not apparent in the current-dollar values and prices.

BEA is currently examining GDP components for residual seasonality, which may lead to improved seasonal adjustment methods. For example, BEA has recently recognized the possibility of residual seasonality in its measure of federal government defense services spending. Also, BEA is testing for seasonality in a number of not-seasonally-adjusted series from the Census Bureau’s quarterly services survey that now have sufficient time spans to which seasonal adjustment techniques can be applied.  If necessary, improvements to the seasonal adjustment methods for these series will be introduced as part of the regular annual revision to the national income and product accounts, scheduled for release in July 2015.


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