Convenience sampling is a non-probability sampling where the sample is selected from the part of the population that is near to hand. I.e. the sample is selected because it is convenient and readily available. Convenience sampling is also known as “grab sampling” or “opportunity sampling”.
In the vast majority of situations, an auditor cannot draw conclusions about the total population from a convenience sampling approach, because it is not likely to be representative of the population.
For example, if an auditor is agreeing an audit sample of invoices to the purchase ledger, to ensure the amounts/names/etc. agree, they may be tempted to choose the invoices that are on the purchase ledger clerk’s desk, and it doesn’t involve trawling through filing cabinets. This would be an inappropriate use of convenience sampling because:
- The clerk may have known the auditor was coming and ensured that those invoices close at hand were all correct
- The invoices close to hand are likely to be recent, and if the clerk knew an audit was imminent may have ensured all recent invoices were correct
- The invoices close to hand may only be from a few suppliers, and hence this sample wouldn’t identify problems with other suppliers
- The clerk you are speaking to may only deal with part of the ledger (certain suppliers)
In this situation, it may be more appropriate to use a random sample or use probability proportional to size sampling.
Another example of poor use of convenience sampling is when attending a stock take. If the auditor only tests those items which are easy to count (i.e. are close by, or there are only a few items) then there is the risk that the counters will count those that are easy to count, but make up numbers for those that are harder to count.
In this case, the auditor should pick a random audit sample of items from the inventory listing (to agree from book to floor), and a random sample of inventory locations (to agree from floor to book).
One situation where it may be appropriate for an auditor to use convenience sampling is when performing a walkthrough to test IT controls. Where an IT system performs certain controls and the auditor can be sure that if the controls work once they will always work as specified, it may be acceptable to pick an item close to hand to use for the sample.
In general, auditors should be very careful of using convenience sampling, and where they do use it they should ask themselves
- Is there a good reason that a particular convenience sample should be representative of the population?
- Would a random sample from the population deliver the same conclusions as a convenience sample?
- Can the question being asked by the auditor be adequately answered by convenience sampling?