2. Software Reliability
It is defined as the ability of a system or component to perform its required functions under stated conditions for a specified period of time.
Software may retire only if it becomes obsolete. Some contributing factors are – change in environment, infrastructure/tech, increase in complexity, slow execution speed.

3. It is the probability of failure free operation of a program for a specified time in a specified environment.

4. Software and Hardware Reliability
Failure usually occurs only when a program (design) exposed to an environment that it was not developed or tested for. H/w The probability of failure due to wear and other physical causes has usually been much greater than due to un recognized problem. It is possible to keep H/w design failures low because the h/w is less complex than s/w.

5. Failures and faults
A fault is a defect in a program that when executed in a particular conditions, causes failure. Fault can be a source for more than one failure .
Fault is a property of the program rather than a property of its execution or behavior.
The term failure relates to behavior of the program.

6. A Reliability Model This model focuses on failure intensity.
It assumes that failure intensity decreases with time, that is as execution time increases , the failure intensity decreases.
If the failure is observed , the fault that failure is detected and the fault is removed. Consequently the failure intensity decreases.

7. λ ( µ ) = λo (1- µ / Vo)
λo- initial failure intensity at start of execution
µ- expected no of failures at time t
Vo- total no of failures that will occur in infinite time.
Failure λo
Intensity
Total Failures Vo

8. R(t)= e raise to the power – λτ
R= Realibility, τ= Execution time
µ (τ) = Vo (1-e rtp – λo/ Vo* τ)
Putting value in eq 1
λ (τ ) = λo * e rtp – λo/ Vo* τ
Failure
Intensity
Execution time