Abstract—This paper is mainly focused on the study of the techniques available for the safety analysis of critical systems. It is never possible to build a completely safe system. There is a possibility to bring the behavior of these systems within acceptable limits. For safety evaluation of such systems both formal and informal techniques are available. Both techniques have their own prospects and consequences. Informal techniques are simpler to learn and easier to interpret and have more space for creativity and imagination of the analyst. Formal techniques due to their rigorousness ensure completeness. In this paper, we have analyzed both techniques after defining few parameters. Our study found it that formal techniques are better but usage of informal techniques can never be overlooked. Some approaches combine formal and informal techniques to reap the benefits of both. In some cases, informal techniques can be used as pre-requisite to narrow down the input of minimal critical set for formal techniques and reduce the effort required for formalization of the entire system. Index Terms—Formal techniques, informal techniques, safety analysis, safety critical systems, fault trees. I. INTRODUCTION A safety critical system is one whose malfunctioning may result in loss of human lives or some serious injury, severe damage or loss to some expensive and sensitive equipment or leakage of pollutants or nuclear radiations and wastes which may harm the environment badly [5]. Safety is internal property of a system but safe system can never be guaranteed. However, if in some system risk of damage to life, environment or property can be controlled and brought within the acceptable limits, then such a system can be called safe. With continually increasing penetration of IT into industry and service sectors, numbers of critical systems are increasing and there is more demand for safer systems [3], [11]. This paper critically analysis all such techniques (formal or informal). A wide variety of techniques for safety analysis is available. These techniques are both formal and informal such as Fault Hazard Assessment (FHA) [12], Fault Tree Analysis (FTA) [15], Failure Mode Effect Analysis (FMEA) [21] and FSSA [8] and DCCA [19]. Formal techniques are currently focus of the academic research because these techniques keenly consider the system under analysis and have bright prospects to ensure safe systems. Many newer formal approaches for safety analysis are developed [4]. Formal methods have higher probability of providing better Manuscript received September 19, 2012; revised October 14, 2012. The authors are with Center for Software Dependability, Muhammad Ali Jinnah University (MAJU), Islamabad, Pakistan (email: aftab775@yahoo.com, anadeem@jinnah.edu.pk). safety analysis for critical systems [4], [8]. If formal and informal approaches are used in combination, it is definitely an important step towards safer software systems. If Formal methods stress thorough and in-depth analysis then informal methods have greater scope for intuition and imagination by various stakeholders [9]-[11], e.g., possible hazards or errors imagination. Combination of formal and informal techniques can be constructively used for safety analysis. II. ANALYSIS OF SAFETY APPROACHES In this paper, we have analyzed both formal and informal techniques as under; A. Informal (Traditional Techniques) FMEA or FMECA is a bottom up approach where all possible errors are enlisted and then classified to target those errors that may cause hazards or mishaps. FMEA is definitely useful to propose changes in a system during its development, thereby reducing the cost to be incurred if these errors and there consequences were overlooked. All errors are presented in tabular form and are helpful to study the system, its event and safety concerns. Finding out all such errors is a hectic effort and can never be ensured that all the errors are found. Success of FMEA depends upon thorough understating of the system. Alternatively, Fault trees can be automated to reduce the time required to carry out the safety analysis and eventually reduce the error or manual shortcomings of conventional FMEA. The tool that uses model for generation of fault trees, are discussed in [27], can be refined to improve the generation of quality trees. Formalization of FMEA is possible and it should be made a necessary part of comprehensive and detailed safety analysis to improve its quality aspects. FFMEA can be used standalone without combination of other safety techniques for successful and complete analysis of safety critical systems [21], [18]. Despite its several advantages FMEA has a vital drawback of intuitively finding out errors. Imagination of analysts in carrying out safety analysis is quite important. Secondly there might be many errors that are harmless and do not lead to mishaps. Such errors are given undue attention in this technique. This effort can be seen as an overhead and has no role in value addition to the safety of the system. Then FMEA is of course an error prone technique as “human is err” and has limitations to analyze the safety concerns. Out of existing traditional techniques, FMEA is considered better [15] [17]. FTA is a top down approach, in which all possible hazards are enlisted, for a safety critical system and then conventional method of fault trees is used to found all those errors that are root cause of these hazards. Since hazards that may occur are definitely few as compared to the errors in the system, FTA has more probability of success then FMEA but it does not A Survey of Safety Analysis Techniques for Safety Critical Systems Aftab Ali Haider and Aamer Nadeem International Journal of Future Computer and Communication, Vol. 2, No. 2, April 2013 DOI: 10.7763/IJFCC.2013.V2.137 134