\label{sec:chap3}

\section{Historical Origins of FMEA}
\subsection{FMEA designed for simple electro-mechanical systems}
So its old and prob out of date

\subsection{FMEA does not support modularity.}
It is a common practise in industry to buy in sub-systems, especially sensors.
Most sensor systems now are `smart', that is to say, they contain programatic elemnts
even if they supply analog signals. For instance a liquid level sensor that
supplies a {\ft} output, would have been typically have been implemented
in analog electronics before the 1980s. After that time, it would be common to use a micro-processor
based system to perform the functions of reading the sensor and converting it to a current (\ft) output.
For the non-safety critical systems integrator this brings with it the advantages
that come with using a digital system (increased accuracy, self checking and  ease of
calibration etc). For a safety critical systems integrator this can be very problematic when it
comes to approvals. Even if the sensor manufacturer will let you see the internal workings and software
we have a problem with tracing the FMEA reasoning through the sensor, through the sensors software
and then though the system being integrated.
This problem is compounded by the fact that traditional FMEA cannot integrate software into FMEA models~\cite{sfmea,safeware}.
\section{Reasoning Distance}
\section{Comparison Complexity}



\section{FMEA - General Criticism}

\subsection{FMEA - General Criticism}

\begin{itemize}
   \item FMEA type methodologies were designed for simple electro-mechanical systems of the 1940's to 1960's.
   \item Reasoning Distance - component failure to system level symptom
   \item State explosion - impossible to perform rigorously
   \item Difficult to re-use previous analysis work
   \item Very Difficult to model simultaneous failures.
  
\end{itemize}

%




\subsection{FMEA - Better Methodology - Wish List}
 

\subsection{FMEA - Better Metodology - Wish List}

\begin{itemize}
  
    \item State explosion  
   \item  Rigorous (total coverage)
    \item Reasoning Traceable  
    \item Re-useable
  \item Simultaneous failures
  % \item  
\end{itemize}

%FMEDA is a modern extension of FMEA, in that it will allow for 
%self checking features, and provides detailed recommendations for computer/software architecture,
%but