Detailed description of RCA techniques
C.6 Why-Because Analysis (WBA) 1051
whether A is a necessary causal factor of B. Suppose two events or situations A and B have 1056
been observed. The CT asks whether, had A not occurred, B would also not have occurred.
1057
(Since A did occur, a supposition that A had not occurred is contrary to fact, hence the word 1058
“counterfactual”.) In asking this question all other conditions are assumed to have remained 1059
the same. If the answer is yes: B would not have occurred then A is a necessary causal factor 1060
of B. If the answer is no: B could have happened anyway even if A had not happened ( the CT 1061
fails) then A is not a necessary causal factor of B.
1062
The network of causal-factors is displayed as a Why-Because Graph (WBG), a collection of 1063
“nodes”, boxes, diamonds and other shapes, containing a brief descript ion of the fact, joined 1064
by “edges”, or arrows, where the node at the tail of an arrow is a necessary causal factor of 1065
the node at its head, as determined by the CT.
1066
A WBA is acyclic (contains no loops), so is usually drawn with arrows pointing in the gener ally 1067
upwards direction, as above, or horizontally with arrows pointing generally left -to-right, or 1068
right-to-left.
1069
In order to determine whether sufficiently causal factors are present in the collection of events 1070
and situations presented, the Causal Complet eness Test (CCT) is used. The CCT is applied 1071
to a given event or situation and its collection of necessary causal factors as determined by 1072
the CT. If the CCT is not passed, then the collection of events and situations must be 1073
extended by further factors until it is passed. Suppose A1, A2,...,An have been determined to 1074
be necessary causal factors of B by the CT. Then the CCT is deemed to be passed if, had B 1075
not occurred, one of A1, A2,...,An would not have occurred either (formally, NOT -B is a 1076
necessary causal factor of NOT( A1 AND A2 AND...AND An) as determined by the CT).
1077
When a WBG has been constructed and the CCT is passed for all the events and situations 1078
therein, then the WBG is complete and is deemed to represent a sufficient causal explanation 1079
of the focus event.
1080
Figure C.7 illustrates an example of a WBG for a commercial-aviation runway-overrun 1081
accident.
1082 1083
(11) aircraft hits earth bank
(13) earth bank in overrun path
(16) built by airport authority for radio
equipment
(12) aircraft overuns runway
(15) unstabilized
approach
(18) crew’s actions
(14) braking delayed
(17) wheel braking
delayed
(20) aquaplaning
(25) low weight on each
main gear wheel
(30) excessive speed on
landing (27)
condition of runway surface
(24) runway very wet
(28) weather conditions
(29) amount of water on
runway surface
(34)
crews actions in expectation of windshear
(19) speed brakes & thrust reversers deployment
delayed
(23) braking systems
logical design
(22) actual landing
(21) divergence between consequences of design
& behaviour expected by crew
(26) behaviour expected by
crew
(33)
‘normal’ behaviour expected by crew (32)
company X procedures (31)
crews training at company X
1084
Figure C.7 – Example of a WBG 1085
C.6.2 Process 1086
WBA has the following steps.
1087
1) Determine a collection of facts deemed to be rel evant, under guidance of a stopping rule.
1088
This gives an initial collection C of facts, divided into events, states and situations.
1089
2) Select the focus event (called in WBA the Accident Event) F.
1090
3) Determine intuitively the immediate necessary causal factors of F from amongst the 1091
collection C; check using the CT. Display the results visually as a partial WBG.
1092
4) Determine intuitively the necessary causal factors of those immediate factors; check using 1093
the CT. Extend the WBG with these factors.
1094
5) Proceed to fill out the analysis (to extend the WBG) by testing each fact in C against the 1095
factors already in the WBG.
1096
6) Apply the CCT to determine whether the WBG is complete, or whether factors are missing 1097
from the collection C.
1098
7) Extend C if necessary; incorporate the new facts into the WBG using the CT. If insufficient 1099
information is available, assumptions may be included, providing they are clearly labelled 1100
as such.
1101
8) Finish when the CCT shows sufficient causal factors for each factor, under cognisance of 1102
the stopping rule. If insufficient facts are available, assumptions must be included in order 1103
to allow the CCT to succeed, but clearly labelled as such.
1104
C.6.3 Strengths and limitations 1105
The strengths of WBA are as follows:
1106
may be performed with a minimum of training (with the use of suitable tools that provide 1107
help on extracting facts from narrative descriptions, an inexperienced analyst can typically 1108
perform a first pass WBA inside two hours);
1109
the analysis results are easily understandable by third parties ; 1110
the conceptual background required to perform a WBA is limited (an analyst must be able 1111
to apply the CT, and then the CCT);
1112
any network of causally-related phenomena may be analysed with a WBA;
1113
the reasoning behind a WBA may be formally checked using a formal logic;
1114
can be used together with other methods; say, those providing more structure to the 1115
collection of facts.
1116
The limitations of WBA are as follows:
1117
the method provides no guidance on the collection of facts to which the tests are applied 1118
e.g. there is no structuring of facts into categories, for example technical, procedural, 1119
human-factors, organisational and legal;
1120
because facts are not structured, WBA provides limited guidance on corrective action, in 1121
the case recurrence is to be prevented.
1122
C.7 Fault Tree and Success Tree Method