Audits of T&E are performed in various ways by depending on the company's volume of T&E, philosophy on its controls over it, whether the company is public or private, and industry practices.
As anecdotal evidence we have heard of companies performing 100% audits on certain types of T&E transactions or over certain dollar amounts. Others use statistical sampling of their T&E based on sample sizes that would be relevant and represent the population of T&E dollars. Keep in mind to focus on certain transactions that lend themselves to errors, omissions, or outright intentional fraud such as inflating cash tips, cab receipts submitted more than once, hotel receipts with dates changed...
Below are resources for your audits:
- White paper by the IBM Thomas J Watson Research Center and its Global Technology Services that describes techniques for business controls with T&E.
- Sample Size Calculator in the TAPN tools section under AP Accounts Forms and Checklist Templates.
- Audit Proofing Strategies
Your question is also a good one for the Messaging/Forum section at TAPN where you can ask your peers how they handle their audits and if they break them down into discrete dollar ranges and the frequency.