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Within the chemical industry it has become common practiceto assign a so-called SIL-value, which stands for safetyintegrity level and which is part of the international IEC61508 standard. It is an expression of a barrier’s assumedreliability on a three-point scale. The number actually reflectsthe exponent x in the formula 10−x rounded-off to the lowestinteger. So a SIL-value of 3 corresponds to a probabilityof failure on demand between 10−4 and 10−3. TheM-index resulting from the ARAMIS audit can be used tomodify the SIL-values of any barrier in any scenario underinvestigation.The procedure for calculating the M-index is still ratherexperimental, in that it is not based on empirical evidenceon management influences or experience. First of all, a decisionhas been made that the delivery systems ‘risk analysisand barrier selection’ and ‘learning and change management’should be excluded from the quantification phase (see above).The remaining seven delivery systems however do contributeto current barrier effectiveness.These delivery systems first had to be brought back to onesingle number, instead of a separate rating per step. Therefore,several persons with a certain degree of expertise (somemembers from the ARAMIS consortium and 13 delegatesfrom a master’s course in safety) judged all steps withinthese delivery systems for their influence on barrier effectiveness.This resulted in a split between delivery systemshaving equally weighted steps and systems having unequallyweighted steps. For the equally weighted delivery systems,a geometrical mean is calculated to arrive at their overallnumerical rating (please note that all steps have been ratedon a five-point scale). For the unequally weighted ones, however,a different reasoning applies. An average is first taken,but when steps within a particular delivery system are judgedmore important than others, the final mean rating of such asystem cannot be higher than the lowest scoring importantstep. Or put in other words, the quality of less importantsteps cannot compensate for the lesser quality of importantsteps
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