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\section{INTRODUCTION}   Reliability is typically  defined as the ability of a system to serve a given function over time. The In this sense the  reliability of the a  particle accelerator is very important for user facilities, facilities  like storage ringbased  light sources;  but it is as well sources  and provides  an important design objective for newtypes of facilities as accelerator driven systems or   new  large scale accelerator  facilities as for example the International Linear Collider~\cite{Himel_2007}. An Clearly  operation metrics should quantify the reliability of a particle accelerator. If the objective is to assess the improvement of a specific facility over time,   then the operation metrics metric  should bevery  closely related tothe  specific user requirements for that facility~\cite{Luedeke:2014aa}.  But in requirements~\cite{Luedeke:2014aa}.  In  order to comparethe  reliability of different facilities facilities,  one needs a common standard for the calculation ofthe  operation metrics. In particular Commonly  beam availability or "up-time"  is often used to compare the reliability of light sources. While these statistics are published for most light sources,   few facilities do supply definitions on how these numbers are calculated.   A survey of several light sources revealed~\cite{L_deke_2009}