Sometimes an auditor simply doesn't have the specialist knowledge needed to audit a particular area — and that's perfectly fine. SA 620 tells you exactly what to do when you bring in an outside expert to help gather audit evidence. Think of it this way: Rajesh & Co. is auditing a real-estate developer with a large land bank. The auditor cannot personally value 50 acres of farmland in Nashik — so he brings in a registered valuer. That expert is the auditor's expert under SA 620.
The single most important rule in SA 620: using an expert does not reduce the auditor's responsibility one bit. The audit opinion still belongs to the auditor. The standard draws a sharp line between two types of expert. An internal expert works within the audit firm (e.g., an in-house actuary). An external expert is hired from outside. Both are covered by SA 620 — but a management-appointed valuer or actuary is a management's expert and falls under SA 500 (audit evidence), not SA 620. Students mix these up constantly in exams.
Before relying on the expert's work, the auditor must assess three things: competence (does the person have the right qualifications and experience?), capability (can they do this specific job right now — time, resources, access?), and objectivity (are there any conflicts of interest?). These three words appear verbatim in exam questions — memorise them together.
The auditor must also agree the scope of work with the expert — what needs to be done, the methods to be used, the assumptions, and what the final output will look like. Finally, the auditor evaluates the adequacy of the expert's work: Are the assumptions reasonable? Is the source data reliable? Does the conclusion make sense in the context of the audit? If the auditor is not satisfied, they negotiate with the expert, perform additional procedures, or — in extreme cases — modify their opinion. This section is asked frequently as a 4-mark or 6-mark theory question in Paper 5.