Detailed description of RCA techniques
C.5 Causes Tree Method (CTM) 978
C.5.1 Overview 979
CTM [12] is a systematic technique for analysing and graphically depicting the events and 980
conditions that contributed to a focus event.
981
The method examines all the system components associated with the focus event. The 982
investigation starts by establishing the tangible facts, taking care, in this phase, not to 983
interpret them or to express an opinion about them.
984
CTM is similar to the why method in concept but builds a more complex tree and explicitly 985
considers technical, organisational, human and environmental causes. Each antecedent 986
CTM is also similar to a fault tree but whereas a fault tree is used prior to an event to explore 990
all possible causes and strict logic relationship(s) between faults are specified, the cause tree 991
includes only those causes which apply to a specific event that has already occurred, and 992
does not develop the logical relationships in detail.
993
A cause tree may be used to explore successes as well as failures . 994
The Cause Tree forms a network of the causes which have directly or indirectly caused the 995
focus event, using the three logical relations shown in Figure C.5.
996
Sequence: A cause (Y) has a single direct origin (X). i.e. (X) was necessary and sufficient for (Y) to occur.
Conjunction A cause (Y) has several direct origins (X1) and (X2). That means, each of the direct origins (X1) and (X2) was
necessary for (Y) to occur.
Disjunction or Separation: Two or more causes (Y1;Y2, …) have a single and identical direct origin (X). That means (X) was necessary for (Y1) and (Y2) to be produced.
Figure C.5 – Symbols and links used in CTM 997
Figure C.6 shows an example tree, in which Mr L (the victim) and Mr A are working nights, as 998
an exception, to store a surplus of cooked matter stock. In accordance with the handbook, Mr 999
L and Mr A were required to load the cooked matter crusher with "flour" which is then bagged 1000
and stored. Normally this activity is under the responsibility of a head of team whose 1001
presence had not been considered essential by the management for this night. Of his own 1002
initiative to save time Mr L took a forklift truck (the ignition key had remained on the 1003
dashboard as usual) to store the bags. At the end of the task, Mr L set about returning the 1004
forklift truck. Mr L carried out a sharp bend in reverse, forks raised and while seeking to avoid 1005
a hole on the ground the forklift tipped over, crushing Mr L between the ground and the right 1006
safety upright of the truck.
1007
X Y
Y X1
X2
X
Y1
Y2
L & A decide to borrow the forklift
truck
L uses the forklift truck for storing the
bags L & A have finished storing the
bags (end of job)
L puts the forklift truck away L puts the forklift truck back where it
was
L drives the truck only very occasionally
L reverses with the forks raised L turns while
reversing L drives round the
hole in the floor A hole is present in
the path of the truck
The truck tips over L is ejected from the truck
L is on the floor
L is in the path of the upright
L is crushed between the floor and the right safety upright of the truck
The right upright of the truck is present
1008
Figure C.6 – Example of a Cause Tree 1009
C.5.2 Process 1010
CTM has the following steps.
1011
1) Identify the focus event to be analysed and record as the starting point for the tree . 1012
2) Collect and record all relevant data including people, their activities, and actions, 1013
materials and equipment and factors relating to the physical and psychosocial 1014
environment.
1015
3) Make a list of the causal factors to the focus event. These should be supported by 1016
evidence and be expressed as precisely as possible. Subjective opinions and judgements 1017
are not included. Causal factors include those which are unusual or a change from the 1018
normal course of events and those which are a normal but played an active part in the 1019
occurrence of the event.
1020
4) Work backwards towards the root causes by asking the following questions systematic ally 1021
for each antecedent that has been gathered.
1022
a) What antecedent X has directly caused the antecedent Y?
1023
b) Was X in itself necessary to give rise to Y?
1024
c) If not, what are the other antecedents (X1, X2...) that were equally necessary in order 1025
to give rise directly to Y?
1026
5) Display these immediate necessary causal factors in a box linked by an arrow to the focus 1027
event. (The tree may be drawn horizontally or vertically but is normally drawn horizontally 1028
starting from the right, so that left to right corresponds to the chronology of events).
1029
6) Continue asking the same questions with respect to each cause found until the team 1030
agrees that there is no value in continuing further.
1031
7) Check the validity of the tree by obtaining further evidence that determines whether it is 1032
true.
1033
C.5.3 Strengths and limitations 1034
The strengths of CTM are as follows:
1035
provides a method for structuring investigation of complex events ; 1036
orderly easy to read format;
1037
can be used to encourage group participation ; 1038
identifies areas for collecting data as the in vestigation proceeds;
1039
can be used to analyse success or failure events ; 1040
can be used for technical and non-technical events.
1041
The limitations of CTM are as follows:
1042
many human and organisational factors may contribute to the occurrence of the focus 1043
event and it is often difficult to establish which in a particular instance were the necessary 1044
causal factors;
1045
there is no guidance on how to seek causes, therefore expertise in human error and 1046
organisational systems is needed when the tree involves human and organisational 1047
failures, where evidence is often difficult to obtain ; 1048
it is difficult to apply when an event occurs as a result in a change of quality in several 1049
areas, where no single cause is a necessary cause.
1050
C.6 Why-Because Analysis (WBA)