In January 2021, the Federal Reserve Bank of Philadelphia published its annual historical data revisions that incorporated new seasonal factors using data through December 2020. However, those estimated seasonal factors were significantly impacted by the onset of the pandemic in ways that are not typical of seasonal trends.

To remove the effect of the pandemic-induced volatility from our seasonal adjustment process, we are re-using the factors that were calculated in December 2019 for the 2020 data and applying those factors to the 2021 data as well.

As a result, in April 2021, we revised the historical data, including the data previously published for January 2021 through March 2021. For most indexes, the revisions resulted in modest changes, and general trends are comparable with what was originally published.

For example, the revisions show elevated readings for the current general activity index and a similar pattern for the first three months of 2021 compared with the data we had released in March; however, the index readings are slightly higher for January and February and lower for March than originally published (see chart below).

Current General Activity Index Revised vs. Original Release

Seasonal adjustments were based on the U.S. Census Bureau's Census X-12 program for 50 years of data through December 2019, and seasonal factors for the 12 months of 2021 repeat the factors for the 12 months of 2020, which were projected in January 2020 using the X-12-ARIMA method.

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