exist that indicate that we will not recover our initial investment. Impairment charges are recorded on investments having a cost basis that is greater than the value that we would reasonably expect to receive in an arm’s length sale  of the investment. For 2002, 2001 and 2000, we recorded write-downs of our cost basis investments of $2,010,182, $5,528,930 and $1,182,984, respectively. These write-downs are included in net realized investment gains (losses)  in the accompanying consolidated statements of income. Results of Operations  The following table presents selected financial information derived from ADTRAN’s consolidated statements of income expressed as a percentage of sales for the years indicated. (Stated as % of sales) Years Ended December 31, 2002 2001 2000)) Sales Carrier Networks Division 63.3% 61.6% 68.1% Enterprise Networks Division 36.7 ))) 38.4 ))) 31.9 )v Total sales 100.0))) 100.0))) 100.0))) Cost of sales 49.4 ))) 55.2 ))) 50.4 ))) Gross profit 50.6))) 44.8))) 49.6))) Selling, general, and administrative expenses 23.5 ))) 24.8 ))) 18.8 ))) Research and development expenses 16.2 ))) 15.2 ))) 10.9 ))) Operating income 10.9))) 4.8))) 19.9))) Interest income 2.6 ))) 2.2 ))) 1.9 ))) Interest expenses (0.7) )) (0.5) )) (0.4) )) Other expenses 0.0 ))) (0.1) )) 0.0 ))) Net realized investment gains (losses) (3.5) )) (0.2) )) 18.1 ))) Income before provision for income taxes 9.3))) 6.2))) 39.5))) Provision for income taxes  2.1 ))) 1.7 ))) 13.4 ))) Net income 7.2% 4.5% 26.1% 2002 Compared to 2001 Sales ADTRAN’s sales decreased 10.7% from $387,081,000 in 2001 to $345,725,000 in 2002. The decrease was primarily  the result of decreased spending by our customers, which we believe to be a result of both economic and industry-wide factors. In particular, the decrease in overall sales is attributable to a decrease in sales of our Digital Business Transport (DBT)/Total Reach$^{®}$‚ and High-bit-rate Digital Subscriber Line (HDSL)/T1 products, partially offset by increased sales  of our Systems products. Carrier Networks sales decreased 8.2% from $238,367,000 in 2001 to $218,912,000 in 2002. Carrier Networks sales, as a percentage of total sales, increased from 61.6% in 2001 to 63.3% in 2002. Enterprise Networks sales decreased 14.7% from $148,714,000 in 2001 to $126,813,000 in 2002. Enterprise Networks sales, as a per- centage of total sales, decreased from 38.4% in 2001 to 36.7% in 2002. Foreign sales increased 14.9% from $17,658,000 in 2001 to $20,296,000 in 2002. The increase in foreign sales is attributable to market acceptance of the Carrier Networks Division’s Total Access System, which includes the Total Access ® 3000 and SHDSL (Symmetrical HDSL). Cost of Sales Cost of sales decreased 20.1% from $213,760,000 in 2001 to $170,790,000 in 2002. The cost of sales decrease is  primarily related to the decrease in revenues in each respective period and timing differences between the recognition  of cost reductions and the lowering of product selling prices. As a percentage of sales, cost of sales decreased from 55.2% in 2001 to 49.4% in 2002 and is primarily attributable to product cost reductions, in excess of sales price reductions, in the Carrier Networks Division. Carrier Networks cost of sales, as a percent of division sales, decreased from 60.7% in 2001 to 51.9% in 2002. Enterprise Networks cost of sales, as a percent of division sales, decreased from 46.4% in 2001  to 45.1% in 2002. 13 Financial Results