However, 17 invoices are between 90 and 120 days past due, and 13 invoices are over 120 days past due. The auditor should review past-due balances with the credit manager to obtain information for basing an opinion on their collectability. The auditor’s objective is not to assess the collectability of each account, but to determine that the methods used by the credit manager to estimate the allowance for doubtful accounts is adequate and that the overall allowance is reasonable.