With direct write off, one writes off amounts from sales in the past are determined to be uncollectable. An estimate writes off estimated amounts of current sales that are expected to be uncollectable. Does that help you determine the accounting principle used for estimates?

And a fixed asset question: when an asset is fully depreciated (think Kate's Cards after four years when the equipment with cost of $4,800 has accumulated depreciation of $4,800), should a company keep it on the books? Why or why not?