reliability. What is the difference between hardware & software
Software reliability is the probability that software will provide failure-free operation in a fixed environment for a fixed interval of time. Probability of failure is the probability that the software will fail on the next input selected. Software reliability is typically measured per some unit of time, whereas probability of failure is generally time independent. These two measures can be easily related if you know the frequency with which inputs are executed per unit of time. Mean-time-to-failure is the average interval of time between failures; this is also sometimes referred to as Mean-time-before-failure.
Software reliability tends to change continually during test periods. While hardware reliability may change during certain periods such as initial burn in or the end of useful life however it has a much greater tendency then software value.
Hardware faults are not physical faults whereas software faults are design faults that are harder to visualise, classify, detect and correct. In reality, the division between hardware and software reliability is somewhat artificial. Both may be defined in the same way therefore one may combine hardware and software reliability to get system reliability. The source of failure in hardware has generally being physically deterioration.