The direct write-off method records bad debts expense only when an account becomes uncollectible, which is not always in the same period as the sale. For this reason, the direct write-off method violates the principle.

The correct word for the blank space is: expense recognition principle.

Explanation:

The expense recognition principle establishes when expenses and revenues must be recorded in the accounting books of a company. Under the accrual basis of accounting method, revenues are recognized when earned and expenses whenever consumed. While using the cash basis accounting method, revenues are recognized when earned and expenses when they are paid to suppliers not when invoices are sent.

In both cases, expenses and revenues are recognized during the same period when they take place. That is the reason why the direct write-off method fails to fulfill the expense recognition principle because bad debt can be recognized as an uncollectible account not necessarily in the same period where the debt instrument was issued.