You have been asked by your firm to examine the payment vouchers of a company to establish the level of errors and then decide whether better results can be obtained through statistical techniques.Required; Why is it not satisfactory to pick vouchers from a limited period?

It is not satisfactory to pick vouchers from a limited period as these vouchers may not be representative of all the vouchers that were raised in that period.
This is because these vouchers may relate to payments that are subjected to special conditions or conditions that were not prevalent in the company throughout the period, e.g. there could have been a different accountant recording the payments made via those vouchers different people giving authority etc.