. depreciation is computed using the straight-line method based on average estimated useful lives , determined principally in reference to associated product life cycles , primarily five years . we review instruments for impairment in accordance with sfas no . 144 . depreciation of instruments is recognized as selling , general and administrative expense . goodwill 2013 we account for goodwill in accordance with sfas no . 142 , 201cgoodwill and other intangible assets 201d