False Acceptance Rate (FAR, Type II error)
The rate at which a biometric system wrongly accepts an impostor, commonly called a Type II error. Of the two biometric errors it is the one that admits the wrong person.
The false acceptance rate measures how often a biometric system matches an impostor’s sample to someone else’s enrolled template and grants access. Study sources commonly call a false acceptance a Type II error. NIST SP 800-63B-4 uses the corresponding matching term, false match rate, and notes that the chosen threshold usually emphasises a low false match rate to maximise security. It is the error that admits the wrong person.
FAR is not tuned in isolation. Raising the matching threshold makes the system stricter, which lowers false acceptances and raises the false rejection rate; relaxing it does the reverse. The point where the two rates meet is the crossover error rate. A low FAR also covers only chance matches. A deliberate spoof, such as a copied fingerprint or a photograph, is a presentation attack, which SP 800-63B-4 treats as a separate risk from matching error. That is one reason the same document allows a biometric only as part of multi-factor authentication with a physical authenticator.
Exam relevance: a scenario is likely to describe an unauthorised person admitted by a fingerprint or face reader and ask which error occurred, where false acceptance, Type II, is the expected label. Candidates are expected to know which way FAR moves when the threshold is raised.