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\section{DISCUSSION}  Operation managers need to prioritize maintenance and upgrades, and  to makean  efficient use of the available resources. The effect of these measures on the Trends in  operation metrics is today an are  important criteria to assign priorities to these tasks.  A multi dimensional Multi-dimensional  metrics like theproposed  common operation metrics proposed here  would allow a more comprehensive evaluation ofthe appropriate  priorities. If an upgrade of the injector would help to reduce the number of low-beam-current failures  then a detailed statistics of these failures for the past injector faults  is important to justify the upgrade and a comparison to the low-beam-current failure rate at other facilities could help  to justify the need foran  improvement. Common operation metrics data from other light sources could further help to assess the validity  of a specific measure:  it could demonstrate how similar measures at other facilities did help helped  to improve specific failure modes at those facilities; modes;  and the current failure rates can be put into perspective directly compared  tothose at  other facilities. Existing statistics like "beam availability" may be continued to be used,either  to illustrate how the a  facility developed evolves  over time, or if they are part of contracts. But In addition  it would be possibleat the same time  to calculate values for a standardized definition of ``beam availability''  for at  all facilities that publish their common metrics data. The proposed common metrics does do  require an effort  to record a larger number of failure modes for several operation modes of a at each  facility. An automated recording of those failures would be beneficial  to achievean  accurate and reproducible accounting. The authors consider ``no-beam'', ``low-beam-current'' and ``distorted-orbit'' to   be the most important failure modes of for  storage ring light sources. The detection of ``beam-blow-up'' would be is also  important, but many facilities still lack the equipment to continuously measure the vertical beam size down to a level levels  that matters matter  tothe  users. Every facility should publish the number and duration of ``short-uptime'' failures;  those failures are already detected but if they would be published  it would help for a better facilitate  comparison ofthe  operation statistics from different facilities. light sources.  Some failure modes are considered of secondary priority by the authors. priority.  The ``low-beam-lifetime'' is easy to measure but often   not a significant failure mode for machines running in top-up.  Lifetime limits would be very low and actual eventsso  rare. A ``distorted-fill'' failure is mostly relevant for timing modes;   the bunch purity levels required at some facilities  can often not be measured in parallel with user operation,   which prohibitsan  automatic recording of this failure mode.  And the recording.  The  failure mode ``beam-unrelated'' is too broadly defined for an automated recording. The authors suggest that if certain types of this failure mode   -- like such as  an interlock to the beam line  photon shuttersof the beamlines  -- is occurring at a facility, a specific failure mode for this type   should be recorded and published. recorded.  The authors are aware that successful user beam time depends on many factors, e.g. availability, dependability, etc. such as availability and dependability  of the data taking environment. acquisition system.  Nevertheless the availability of photons of promised property and quantity photon beams with specified parameters  on sample is the commonand unique  prerequisite. This paper is the attempt attempts  to characterize significant quality and quantity losses to the available beam time inan  unambiguous way, ways,  so it they  can be accounted for and compared. This is done from the perspective of a small number of very different light sources. If a number of facilitieswould  publishtheir failure  data according to this proposal, it would at least allow a permit  meaningful comparison comparisons  of the reliability of those facilities. And that would be already reliability,  a significant step forward compared to the current situation. It forward. This  is the a  first attempt tofind a  standardized definition of an definitions for  operation reliability for storage ring light sources, at least to the knowledge of the authors. Certainly specifics of operational modes and resulting failures and their severity have to be completed for every facility. The performance of storage ring light sources has been increasing for the past decades.  Diffraction limited light sources aim now to provide extremely