There are two main metrics that are used to determine the performance of biometrics: 1. FAR (False Acceptance Rate) 2. FRR (False Rejection Rate) False Acceptance Rate (FAR) is a metric for bio-metric performance that determines the number of instances where unauthorized persons were incorrectly authorized. False Rejection Rate (FRR) is a metric that determines the number of instances where an authorized person are incorrectly rejected. If the emphasis is security, then making sure the False Acceptance Rate is low as a low FAR rate means a lower possibility for someone to be authorized who shouldn't. If the emphasis is convenience, then you'd want to make sure the False Rejection Rate is low as a low FRR means a lower possibility for someone to be rejected who should be authorized.