There are two basic methodologies that are commonly used for conducting audits. Horizontal, or just skimming the surface, vertical which is in an in-depth audit and you can also use use a combination of the two, stepwise digging where necessary followed by a return to the surface.
The Horizontal Audit
A horizontal audit involves tracking a particular process from one end to the other, like raw materials through finished product release and distribution.
A process audit will cross a number of interfaces between areas, functions or departments. It is the key to understanding how an organization functions and is best conducted with front-line, operational staff.
Example
As an example, horizontal audits can be conducted on aspects of equipment qualification.
Is the equipment qualified at the appropriate time; is it coded to indicate qualification status; are qualification protocols/ reports/ certificates and records of qualification up-to-date; are all documents readily available at designated location and so on.
Horizontal auditing is a good way to get a feel for the site and its problems.
The Vertical Audit
Vertical auditing, on the other hand, involves checking all aspects of a quality system within a particular area, function or department. A Functional Audit concentrates on processes, procedures and records restricted to the department itself and does not cross inter-departmental boundaries.
Example
For example, annual product reports can be reviewed for number of batches, quality parameters limits, deviations, change control, market complaints and so on. Different areas are examined, normally in a logical sequence working forwards or backwards.
This approach is likely to be somewhat better than the first approach. It has the advantage of covering all of the key processes and their interfaces within the function being audited. It also provides a window on problems at functional boundaries.
The Combination Audit
The ideal Audit would combine both methodologies with mostly horizontal auditing and occasional vertical auditing when possible issues are noted or something does not quote/unquote feel right.