Handbook of Information Security Management:Access Control

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These are the important factors necessary for any effective biometric system: accuracy, speed and throughput rate, acceptability to users, uniqueness of the biometric organ and action, resistance to counterfeiting, reliability, data storage requirements, enrollment time, intrusiveness of data collection, and subject and system contact requirements.


Accuracy is the most critical characteristic of a biometric identifying verification system. If the system cannot accurately separate authentic persons from impostors, it should not even be termed a biometric identification system.

False Reject Rate

The rate, generally stated as a percentage, at which authentic, enrolled persons are rejected as unidentified or unverified persons by a biometric system is termed the false reject rate. False rejection is sometimes called a Type I error. In access control, if the requirement is to keep the “bad guys” out, false rejection is considered the least important error. However, in other biometric applications, it may be the most important error. When used by a bank or retail store to authenticate customer identity and account balance, false rejection means that the transaction or sale (and associated profit) is lost, and the customer becomes upset. Most bankers and retailers are willing to allow a few false accepts as long as there are no false rejects.

False rejections also have a negative effect on throughput, frustrations, and unimpeded operations, because they cause unnecessary delays in personnel movements. An associated problem that is sometimes incorrectly attributed to false rejection is failure to acquire. Failure to acquire occurs when the biometric sensor is not presented with sufficient usable data to make an authentic or impostor decision. Examples include smudged prints on a fingerprint system, improper hand positioning on a hand geometry system, improper alignment on a retina or iris system, or mumbling on a voice system. Subjects cause failure to acquire problems, either accidentally or on purpose.

False Accept Rate

The rate, generally stated as a percentage, at which unenrolled or impostor persons are accepted as authentic, enrolled persons by a biometric system is termed the false accept rate. False acceptance is sometimes called a Type II error. This is usually considered to be the most important error for a biometric access control system.

Crossover Error Rate (CER)

This is also called the equal error rate and is the point, generally stated as a percentage, at which the false rejection rate and the false acceptance rate are equal. This has become the most important measure of biometric system accuracy.

All biometric systems have sensitivity adjustment capability. If false acceptance is not desired, the system can be set to require (nearly) perfect matches of enrollment data and input data. If tested in this configuration, the system can truthfully be stated to achieve a (near) zero false accept rate. If false rejection is not desired, this system can be readjusted to accept input data that only approximate a match with enrollment data. If tested in this configuration, the system can be truthfully stated to achieve a (near) zero false rejection rate. However, the reality is that biometric systems can operate on only one sensitivity setting at a time.

The reality is also that when system sensitivity is set to minimize false acceptance, closely matching data will be spurned, and the false rejection rate will go up significantly. Conversely, when system sensitivity is set to minimize false rejects, the false acceptance rate will go up notably. Thus, the published (i.e., truthful) data tell only part of the story. Actual system accuracy in field operations may even be less than acceptable. This is the situation that created the need for a single measure of biometric system accuracy.

The crossover error rate (CER) provides a single measurement that is fair and impartial in comparing the performance of the various systems. In general, the sensitivity setting that produces the equal error will be close to the setting that will be optimal for field operation of the system. A biometric system that delivers a CER of 2% will be more accurate than a system with a CER of 5%.

Speed and Throughput Rate

The speed and throughput rate are the most important biometric system characteristics. Speed is often related to the data processing capability of the system and is stated as how fast the accept or reject decision is annunciated. In actuality, it relates to the entire authentication procedure: stepping up to the system; inputting the card or PIN (if a verification system); input of the physical data by inserting a hand or finger, aligning an eye, speaking access words, or signing a name; processing and matching of data files; annunciation of the accept or reject decision; and, if a portal system, movement through and closing the door.

Generally accepted standards include a system speed of 5 seconds from startup through decision annunciation. Another standard is a portal throughput rate of 6 to 10/minute, which equates to 6 to 10 seconds/person through the door. Only in recent years have biometric systems become capable of meeting these speed standards, and, even today, some marketed systems do not maintain this rapidity. Slow speed and the resultant waiting lines and movement delays have frequently caused the removal of biometric systems and even the failure of biometric companies.

Acceptability to Users

System acceptability to the people who must use it has been a little noticed but increasingly important factor in biometric identification operations. Initially, when there were few systems, most were of high security and the few users had a high incentive to use the systems; user acceptance was of little interest. In addition, little user threat was seen in fingerprint and hand systems.

Biometric system acceptance occurs when those who must use the system — organizational managers and any union present — all agree that there are assets that need protection, the biometric system effectively controls access to these assets, system usage is not hazardous to the health of the users, system usage does not inordinately impede personnel movement and cause production delays, and the system does not enable management to collect personal or health information about the users. Any of the parties can effect system success or removal. Uncooperative users will overtly or covertly compromise, damage, or sabotage system equipment. The cost of union inclusion of the biometric system in their contracts may become too costly. Moreover, management has the final decision on whether the biometric system benefits outweigh its liabilities.

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