In testing accounts Receivable an auditor sends out positive confirmation request to $75$ randomly

selected customer invoices at December $31$st$.$ The customers returned the confirmation for all the $75$ confirmed receivables with no differences noted between the balances confirmed by the customers and the accounts receivable balances per the client's accounting records. Based on these sampling results, the auditor concludes that the client's account receivable balances are materially correct. However, following the issuance of the financial statements the client discovers an actual material error in accounts receivable at $t_{{?}_{?}}{?}_{?}$ December ${31}^{st}$ in the portion of accounts receivable that were not selected for confirmation by the auditor during the audit.

This is an example of:

Sampling error

Projected misstatement

Standard error

Non sampling error