Audit execution: evidence collection, sampling and interviewing
Evidence and triangulation
Audit evidence comes from interviews, observation of activities and conditions, and review of documents and records. It must be verifiable and recorded precisely: what was seen, where, when, and from whom. Triangulate - if the supervisor says calibration is current, ask for the certificate and check the instrument against it.
Sampling
Auditors rarely examine a whole population; they sample. Judgemental (purposive) sampling targets high-risk, high-volume, recent, or previously problematic items. Statistical sampling supports inferences about the population. Either way, record what you sampled and state the limitation in the report: sampling does not guarantee that no nonconformities exist elsewhere.
Interviewing
Begin with open questions ('walk me through what happens when a customer complains'), listen, then probe with who, what, when and how, following the audit trail. Avoid leading and closed questions early, and avoid interrogating a nervous operator. Interview the level that does the work, not only management. Silence is a tool - let people fill it.
Trap: hearsay and opinion are not evidence.
Trap: one example does not prove the system fails everywhere, but it does prove a nonconformity exists for that instance.
Trap: a well-presented binder is not evidence that the process is followed; verify occurrence, not just documentation.
Rule of thumb
'Show me' beats 'tell me'. If you cannot verify it from a record or a second source, do not present it as a conclusion.