Evaluate the quality of the following pieces of audit evidence, suggesting how each could be used during an audit.

(a) A written assurance from the client’s managing director.

(b) An analysis of standard versus actual costs, prepared by the client's staff.

(c) A report on the value of land and buildings prepared by a firm of chartered surveyors.

(d) An aged analysis of the balances on the client’s debtors’ ledger, prepared by a member of the audit staff.