{
\section{Introduction}
This chapter describes a process for taking a {\fg} of components,
applying FMEA analysis on all the component failure modes possible in that {\fg},
and then determining how that {\fg} can fail.
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With this information, we can treat the {\fg}
as a component in its own right.
This new component, is derived from the {\fg}.
In the field of safety engineering this derived component  corresponds to a low~level sub-system.
%The technique uses a graphical notation, based on Euler\cite{eulerviz} and Constraint diagrams\cite{constraint} to model failure modes and failure mode common symptom collection.  The technique is designed for making building blocks for a hierarchical fault model.
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Once the failure modes have been determined for a sub-system/{\dc},
this {\dc} can be combined with others to form {\fgs} to model higher level sub-systems/{\dcs}. 
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In this way a hierarchy to represent the fault behaviour
of a system can be built from the bottom~up. This process can continue
until there is a complete hierarchy representing the failure mode
behaviour of the entire system under analysis.
%FMMD hierarchy
Using the FMMD technique the hierarchy is built from the bottom up to 
ensure complete failure mode coverage.
Because the process is bottom-up, syntax checking and tracking can ensure that
no component failure mode can be overlooked.
Once a hierarchy is in place, it can be converted into a fault data model.
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From the fault data model, automatic generation
of FTA \cite{nasafta} (Fault Tree Analysis) and mimimal cuts sets \cite{nucfta} are possible. 
Also statistical reliability/probability of failure~on~demand \cite{en61508} and MTTF (Mean Time to Failure) calculations can be produced 
automatically\footnote{Where component failure mode statistics are available \cite{mil1991}}.
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This chapter focuses on the process of converting {\fgs} to {\dcs}, or building the `blocks' of the FMMD hierarchy. 
We can term this stage in FMMD analysis as the `symptom extraction' process. 
The symptom extraction or abstraction process, is the key process in creating an FMMD hierarchy.
}