[Update 1: Clarifies location of Campbell Communications] A new study estimates the $8,000 first-time homebuyer federal tax credit resulted in 357,000 additional home sales so far in 2009. The study, released by Campbell Surveys, a division of Washington, DC-based marketing and public relations firm Campbell Communications, conducted surveys of real estate agents and calculated the figure by comparing the rate of first-time buyers during the first two months of the year both before and after the tax credit was instituted. The monthly rate of first-time buyers increased from 32% in January and February to 43% every other month of the year except July, when it was 42%. Campbell Suveys then, based on the number of homes sold according to research from the National Association of Realtors (NAR) and the US Census Bureau, assigned the number of homes first time buyers purchased and the number of homes above the 32% threshold to arrive at the 357,000 figure. In its own estimate, NAR said the credit brought 350,000 first-time homebuyers into the market. Moody’s Economy.com put the figure at 400,000 in its own study. The tax credit is scheduled to expire on Nov. 30, but legislation introduced in the Senate would extend the credit for an additional six months. Write to Austin Kilgore.