What is the false rejection rate?
The false rejection rate (FRR) measures how often a biometric system refuses a person who is enrolled and entitled to get in. The reader compares the live sample with the stored template, the similarity score falls short of the threshold, and the user sees an error even though the finger, face or eye is the right one. Nobody unauthorized gets in, so FRR is the usability error among the biometric rates. Most security references, including Security+ study material, call a false rejection a Type I error.
How FRR is calculated
FRR equals false rejections divided by genuine attempts. If enrolled staff make 1,500 attempts at a palm-vein reader in a week and 45 are refused, the FRR is 45 / 1,500, or 3 percent. At a site where 300 people badge in four times a day, 1,200 genuine attempts at 3 percent means 36 refused entries daily, each one a person stuck at a locked door or calling security.
Why legitimate users get rejected
- A threshold set strictly to keep the false acceptance rate low.
- Wet, dry, cold, cut or worn fingers; gloves; changes in lighting, glasses, masks or facial hair.
- A poor enrollment sample captured in a hurry on someone's first day.
- Dirty or scratched sensors and badly placed cameras.
- Gradual change from aging or injury.
Some people can't be enrolled at all, which is tracked separately as the failure-to-enroll rate.
Why a high FRR turns into a security problem
Rejections don't let attackers in directly, but they push people toward workarounds. Staff prop open doors that keep refusing them, share badges, or lean on the fallback, such as a PIN or a guard's override, until the fallback becomes the real way in. A phone that rejects its owner's face several times a day trains the owner to type the passcode, and that passcode then carries the device's security.
Common fixes are re-enrolling users with several fingers or face images, cleaning and repositioning sensors, allowing a limited number of retries and choosing a modality that suits the setting, such as face or iris instead of fingerprints where workers wear gloves. Each retry also hands an impostor another try, so retry limits trade FRR against FAR.
FRR vs. FAR vs. CER
FRR and the false acceptance rate (FAR) move in opposite directions as the threshold changes: tighten it and FRR rises while FAR falls. The crossover error rate (CER) is the setting where the two are equal, and it is used to compare products rather than to describe one kind of error. A question about inconvenience to authorized users wants FRR; a question about impostors getting in wants FAR.
False rejection rate on the CompTIA Security+ exam
FRR is not on the SY0-801 acronym list. It is taught with biometrics, one of the authentication methods covered in objective 4.5. A scenario in which enrolled employees are repeatedly turned away, or in which sensitivity was raised and complaints followed, points to a rising FRR. Distractors include FAR, the opposite error, and CER, which a single setting change does not lower. If a question asks how to cut rejections without weakening security, look for better enrollment or a better-suited biometric rather than a looser threshold.