Altering the Seasonal Adjustment Option For Producing Seasonally Adjusted Data For the September 2001 Release

Frequently Asked Questions

Was the option that was altered developed specifically to handle the impact of the terrorist attacks?

No. This option is used as a part of standard seasonal adjustment procedures at the Census Bureau but with a more conservative (restrictive) definition of what constitutes an untypical month. This month, a definition of untypical was used that is customary in statistical practice when there is advance knowledge that the month’s survey value has a strong potential to be untypical.

What was the impact of altering the seasonal adjustment option (additive outlier adjustment) for the September 2001 data month?

All of the kinds of business that the alteration impacted showed a decline from August. For these kinds of business, if the September values had been treated as typical, the program would have taken part of the decline to be a seasonal effect and would have adjusted the data in a way that would have brought the adjusted percent change from August to September closer to zero. The altered option we used determined the declines in these kinds of business to be out of the ordinary. As a result, the August to September percent change in the adjusted value was left further from zero.

How was the survey value for a kind of business determined to be out of the ordinary?

Each month, independent indications of how each kind of business performed are researched as part of our normal review of the estimates to detect reporting errors. This research includes, but is not restricted to, conversations with companies in our survey, a review of publicly available press releases from both companies and industry associations, and a review of other economic indicators.

This month, the September value of a kind of business was considered to be out of the ordinary if it was found to be statistically untypical (as indicated by the appropriate t-statistics having a magnitude larger than a "standard" critical value) and independent sources indicated an impact from the September 11 events.

Did you calculate the August to September percent change without the alteration?

No. The alteration enabled the Census Bureau to produce the best estimates of adjusted sales values and percent changes for July, August, September, and future months.

Could the Census Bureau have overstated the decline?

The alteration affected the adjustments of only those kinds of business that had untypical results based on well-established statistical criteria. Its use minimized the chance of understating or overstating the decline.

How many kinds of business did the alteration affect?

The alteration affected kinds of business accounting for 60 percent of total retail sales.

Do you have an estimate of the impact on wholesale sales and inventories in New York?

No. Our sample is only designed to produce a national estimate.

  

Source: U. S. Census Bureau 
        Service Sector Statistics Division

Last Revised: November 16, 2001