There is no doubt that the reliability of a computer program is an important element of its overall quality. If a program repeatedly and frequently fails to perform, it matters little whether other software quality factors are acceptable.
Software reliability, unlike many other quality factors, can be measured directly and estimated using historical and developmental data. Software reliability is defined in statistical terms as “the probabilit y of failure-free operation of a computer program in a specified environment for a specified t ime”.
Measures of Reliability and Availability
Early work in software reliability attempted to extrapolate the mathematics of hardware reliability theory to the prediction of software reliability. Most hardware-related reliability models are predicated on failure due to wear rather than failure due to design defects. In hardware, failures due to physical wear (e.g., the effects of temperature, corrosion, shock) are more likely than a design-related failure. Unfortunately, the opposite is true for software.
In fact, all software failures can be traced to design or implementation problems; wear does not enter into the picture. There has been an on-going debate over the relationship between key concepts in hardware reliability and their applicability to software. Although an irrefutable link has yet to be established, it is worthwhile to consider a few simple concepts that apply to both system elements.
If we consider a computer-based system, a simple measure of reliability is meantime-between-failure (MTBF): MTBF =MTTF + MTTR where the acronyms MTTF and MTTR are mean-time-to-failure and mean-time-to repair, in addition to a reliability measure, you should also develop a measure of availability. Software availability is the probability that a program is operating according to requirements at a given point in time and is defined as 
The MTBF reliability measure is equally sensitive to MTTF and MTTR. The availability measure is somewhat more sensitive to MTTR, an indirect measure of the maintainability of software.
Software Safety
Software safety is a software quality assurance activity that focuses on the identification and assessment of potential hazards that may affect software negatively and cause an entire system to fail. If hazards can be identified early in the software process, software design features can be specified that will either eliminate or control potential hazards.
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