Human reliability can be defined as the obverse of human error. Human reliability focuses on how likely an individual will perform an error-free task. In other words, human reliability is a function of the task that a person is required to perform in an environment of which failure is predefined. Information of human reliability is important for performance of functional allocation. Variations of human performance can be quantified through calculation of probability for repeatitive discrete tasks or time-continuous tasks.

Repeatitive discrete tasks- the task involve repetition with definite well-defined beginning and ending known as repeatitive discrete task. In such task human error probability (HEP) can be calculated from the following formula:

HEP= number of errors/ number of opportunities of error

In this context, it can be reasonable to state that stress to the participant/ operator may positively influence HEP. In other words, the higher the stress in participant/subject, the higher the HEP.

Time-continuous task- This can be defined as tasks that include vigilance such as monitoring a display along with tracking. Driving of an auto can be modeled according to time to first human error during a defined period (Park, 1987).