When auditing an organization's cash-handling activities, the most reliable form of testimonial evidence comes from a knowledgeable person who is independent of the cashiering duty. This independence ensures that the testimonial evidence is unbiased and not influenced by those directly involved in the processes being reviewed. Such evidence is considered more reliable as it is less likely to be affected by personal interests or biases.
Reference:
The Institute of Internal Auditors (IIA) Practice Guide: Evaluating Evidence IIA Standard 2310 - Identifying Information