As economists track mounting evidence of a recession in the U.S., data released yesterday reveal that a key measure of the market for architectural services, the Architectural Billings Index (ABI), a survey of firms’ billings compiled by the American Institute of Architects (AIA), fell steeply during the month of February—the second tumble in as many months and the largest consecutive decrease in the ABI’s 13-year history.

From its score of 55 in December 2007, the ABI dropped 4.3 points in January, ending the month at 50.7. This was followed in February by an 8.9-point plunge, for a score of 41.8. An ABI number over 50 indicates an increase in billing activity, below 50 represents a decrease. February’s numbers marked the biggest monthly decline since October 2001, when the nation’s economy was last in a recession. Studies suggest that the ABI is a good predictor of construction activity nine months to one year in the future.