incentive compensation expense ( $ 8.2 million ) and related fringe benefit costs ( $ 1.4 million ) , and higher warehousing costs due to customer requirements ( $ 2.0 million ) . corporate overhead for the year ended december 31 , 2006 , increased $ 3.1 million , or 6.5% ( 6.5 % ) , from the year ended december 31 , 2005 . the increase was primarily attributable to higher incentive compensation expense ( $ 2.6 million ) and other increased costs which were not individually significant . other expense , net , decreased $ 2.1 million , or 20.1% ( 20.1 % ) for the year ended december 31 , 2006 compared to the year ended december 31 , 2005 . the decrease was primarily due to a $ 3.1 million decrease in expenses related to the disposals of property , plant and equipment as part of planned disposals in connection with capital projects . partially offsetting the decrease in fixed asset disposal expense was higher legal expenses ( $ 0.5 million ) and increased losses on disposals of storeroom items ( $ 0.4 million ) . interest expense , net and income taxes interest expense , net of interest income , increased by $ 3.1 million , or 11.1% ( 11.1 % ) , for the year ended december 31 , 2006 compared to the full year 2005 , primarily as a result of higher interest expense on our variable rate debt due to higher interest rates . pca 2019s effective tax rate was 35.8% ( 35.8 % ) for the year ended december 31 , 2006 and 40.2% ( 40.2 % ) for the year ended december 31 , 2005 . the lower tax rate in 2006 is primarily due to a larger domestic manufacturer 2019s deduction and a reduction in the texas state tax rate . for both years 2006 and 2005 , tax rates were higher than the federal statutory rate of 35.0% ( 35.0 % ) due to state income taxes . year ended december 31 , 2005 compared to year ended december 31 , 2004 the historical results of operations of pca for the years ended december 31 , 2005 and 2004 are set forth below : for the year ended december 31 , ( in millions ) 2005 2004 change .
what was the operating margin for 2004?
net sales net sales increased by $ 103.6 million , or 5.5% ( 5.5 % ) , for the year ended december 31 , 2005 from the year ended december 31 , 2004 . net sales increased primarily due to increased sales prices and volumes of corrugated products compared to 2004 . total corrugated products volume sold increased 4.2% ( 4.2 % ) to 31.2 billion square feet in 2005 compared to 29.9 billion square feet in 2004 . on a comparable shipment-per-workday basis , corrugated products sales volume increased 4.6% ( 4.6 % ) in 2005 from 2004 . excluding pca 2019s acquisition of midland container in april 2005 , corrugated products volume was 3.0% ( 3.0 % ) higher in 2005 than 2004 and up 3.4% ( 3.4 % ) compared to 2004 on a shipment-per-workday basis . shipments-per-workday is calculated by dividing our total corrugated products volume during the year by the number of workdays within the year . the larger percentage increase was due to the fact that 2005 had one less workday ( 250 days ) , those days not falling on a weekend or holiday , than 2004 ( 251 days ) . containerboard sales volume to external domestic and export customers decreased 12.2% ( 12.2 % ) to 417000 tons for the year ended december 31 , 2005 from 475000 tons in 2004. .