Collecting, cataloging, and using biometric data can be a challenge. This form of data is, like most data, subject to uncertainty and variation. Perhaps unique to biometric data, we see that this information may be affected by changes in an individual's age, environmental influences, disease, stress, occupational factors, training and prompting, intentional alterations, sociocultural aspects of the situation in which the presentation occurs, changes in human interface with the system, and so on.

As a result, each interaction or attempt of an individual may be associated with different biometric information.