Rooting out Causes in Failure Analysis Figure 4: Why Did the Driver Crash at this Bend and Not the One Before?


Road not gritted


Bald tyre


Overgrown hedge


Poor road management


Multiple haemorrhage


No seatbelt


Icy road


Blind corner


Driver died


Through window


Crash


Skidded off road


Swerve to avoid


Saw car too late


Drunk


Depressed


Abused as child


Hit tree (hard)


Driving too fast


Money problems


Sudden decline


The cause of the failure must be found in the difference between the two cases. By restricting attention to the differences between the two cases, you essentially ignore everything they have in common and you dramatically reduce the amount of material and the number of possible causes you need to consider.


By switching through a variety of similar cases, we can generate a large number of hypothetical causes and causal scenarios. Not all of these will be true, but there are well-defined criteria for evaluating causal


By switching through a variety of similar cases, we can generate a large number of hypothetical causes and causal scenarios.


theories and choosing between them. And it is far better to have to choose between too many than to miss the right one or not to have any at all.


Relevance Revisited


Not only does Mill’s difference method root out causes, it ensures the causes it discovers are relevant. Returning to our investigation of the car crash, our forensic psychologist asks “Why this man and not any other man?” and instantly focuses in on the drinking and the driver’s reckless disposition. The road safety expert asks “Why this corner and not the one before?” The meteorologist asks “Why tonight and not last night?” By using different contrasting cases, we filter the causal history to reveal different patterns that emphasise different interests.


This filtering action is illustrated in Figure 4, where instead of simply asking why the driver crashed and died, we ask why he crashed and died on this particular bend and not the one before. The road was icy


88


Left the house late


Argument with wife


Marital problems


on the bend before, and the tyre was just as bald; he was equally drunk and all the antecedent causes to all these factors were equally true. But by asking the contrastive question, we isolate one particular causal chain, which we can then examine to find the relevant cause.


Mill’s Difference Method as a Practical Tool for Failure Analysis


The difference method forms the basis of a powerful tool for discovering relevant causes in cases of machinery and equipment failure. By comparing the case in which the failure has occurred with similar cases in which it has not, we dramatically reduce the field over which we must search for possible causes. Moreover, if a cause can be found in the difference between two real cases then there is a much better chance that it is possible to correct the problem, bringing the problem case closer to the case where the problem has not occurred. Our contrasts need not necessarily be real cases; often contrasting with hypotheticals and expectations can be extremely revealing.


There is much more to fruitful causal analysis than Mill’s difference method. Mill’s method is very good for generating a large number of failure hypotheses: explanations and fragments of explanation. These must be worked up through an evolutionary process in which different explanations both compete and cooperate in order to develop working hypotheses upon which strategies for solution and prevention may be based.


Ultimately, there is no magic handle to turn that automatically churns out a correct diagnosis in a failure analysis. There are many tools and it is important to understand their strengths as well as their limitations and the ways in which they can lead you astray. But with a broad technical insight, a clear conception of the notions of cause and effect and a sound understanding of the criteria for a good explanation, techniques such as Mill’s method can quickly bring investigators close to the relevant factors that not only explain what went wrong, but how to fix it and how to make absolutely sure it never happens again. n


EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 1


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116