Software criticality index

The Software Criticality Index (SCI) refers to the index number obtained from the Software Criticality Level (SCL) matrix. It is essentially the SCL number, an index number (1 through 5) derived from the SCL matrix. This term is intended to replace the older term Software Hazard Risk Index (SHRI), primarily because the word risk in SHRI is misleading for the application. 

The AOP 52 uses in chapter 3 the software safety criticality index (SSCI) to determine the level of rigor that is necessary for testing and analysis that has to be performed by the software and safety development team. It also dictates the level of scrutiny that is necessary during requirements definition, design implementation and validation of safety-related requirements. 

A low index number (i.e., 1) from the software criticality matrix does not mean that a design is unacceptable from a risk or safety standpoint. Rather, it indicates that a more significant level of effort is necessary for the requirements definition, design, implementation, and test of the software and its interactions with the system. 

Software System Hazard Analysis This type of analysis is conducted similar to a hardware system hazard analysis (SHA), analyzing software functional processing steps to determine whether they may have any particular hazardous effect on the system. The analysis utilizes a hazard-risk index to illustrate the severity of each potential failure. The main advantage to this method is in its ability to positively identify safety-critical hardware and software functions as well as consider the effect of the human element in system software operations. The results of the software SHA, which identifies single-point failures or errors within a system, can often be used to assist in the development of a software fault tree analysis or, to some degree, a system FMEA. However, as with the other various SWHA techniques briefly described above, this method is also time-consuming and costly to perform. 

SWOl states that The safety criticality of the software safety requirement is expressed as an index in the range 1 to 5. This index is referred to as the Assurance Evidence Level (AEL). The AEL determines the minimum set of assurance evidence that is required to be available to the regulator for a given software safety requirement for any system proposed for approval. One interpretation of the index provided by SWOl is reproduced below (CAA 2003) ... 

The SCL is an index number ranking the relative safety importance of a software module. This ranking implies that in order to make the software safe, greater development rigor must be applied to each successive criticality level. 

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