Seasonal Adjustment in the PPI
The Producer Price Index (PPI) produces both unadjusted and seasonally adjusted data. Seasonally adjusted data, which removes within-year seasonal patterns from a time series, are computed using seasonal factors. These factors, updated yearly prior to the release of January PPI data, are applied to the current year's data to produce seasonally adjusted indexes and are used to revise the previous five years of seasonally adjusted data. Seasonally adjusted indexes beyond the last five years of data are considered to be final and not subject to revision. The most recent, revised seasonal factors are available on the PPI Tables section of the PPI website.
When to Use Seasonally Adjusted or Unadjusted Data
For analyzing short-term price trends in the economy, seasonally adjusted changes are usually preferred since they eliminate the effect of changes that normally occur at the same time and in about the same magnitude every year— such as price movements resulting from changing climatic conditions, production cycles, model changeovers, holidays, and sales. This allows data users to focus on changes that are not typical for the time of year.
The unadjusted data are of primary interest to data users concerned about prices producers actually receive. Unadjusted data are also used extensively for contract escalation purposes. BLS advises against the use of seasonally adjusted data in escalation agreements because seasonally adjusted series are revised annually.
PPIs Eligible for Seasonal Adjustment
The BLS produces four main types of PPIs: industry net output indexes, industry net input indexes, commodity indexes, and Final Demand-Intermediate Demand (FD-ID) indexes. Of these four types of indexes, only the commodity indexes and FD-ID indexes are eligible for seasonal adjustment. While all FD-ID indexes are eligible for seasonal adjustment, only 3-digit, 4-digit, and 6-digit commodity indexes are eligible for seasonal adjustment.
For the vast majority of commodity indexes, seasonal adjustment is accomplished through direct adjustment. Direct adjustment involves applying seasonal factors to unadjusted data to remove within-year seasonal patterns. Seasonal factors represent the regularly occurring, within-year price pattern of a product.
Additional information on direct seasonal adjustment
In some cases, non-seasonal events, such as natural disasters or wars, can distort the underlying seasonal pattern of an index. In these instances, it may be difficult to determine whether a seasonal pattern exists for a time series and to estimate seasonal factors for the time series. To help overcome this obstacle, PPI employs a procedure known as intervention analysis.
Additional information on intervention analysis
Indirect seasonal adjustment is accomplished by aggregating lower level series components series into higher level indexes. Seasonally adjusted components are used when available (that is, when the lower level index was shown to be seasonal and a seasonal index was calculated); otherwise, unadjusted indexes are used. PPI FD-ID indexes and aggregates of intervention series are indirectly seasonally adjusted.
Last Modified Date: February 12, 2021