Every facility should publish the number and total duration for each applicable primary and secondary failure mode over a year. In addition the user time according to chapter \ref{sec:schedule_statistics} would be published.
These figures would allow a direct comparison of the operational reliability of these light sources.
Each facility could continue to calculate their beam availability according to their own standards, to evaluate the evolution of the reliability of their facility over time. But at the same time the failure mode data would allow to calculate a variety of standardized beam availabilities. A typical example would be an “overall beam availability during user time” and a “compensated beam availability”:
\[\label{eqn:avail} \hbox{Availability}_{\small \hbox{CM-Overall}} = \frac{(T_{\hbox{User}} - \sum{\hbox{no-beam}})}{T_{\hbox{User}}}\]
\[\label{eqn:avail_comp} \hbox{Availability}_{\small \hbox{CM-Compensated}} = \frac{(T_{\hbox{User}} - \sum{\hbox{no-beam}})}{T_{\hbox{Scheduled User}}}\]
Equation \ref{eqn:avail} calculates the beam availability to compare the reliability of different accelerators from the perspective of an accelerator physicist: how reliably did we operate the accelerator? Equation \ref{eqn:avail_comp} is important to compare the facilities from a user perspective: did all scheduled experiments get the promised beam time?
The statistics of the secondary failure modes can be calculated independently, or several failure modes can be convoluted into a combined statistics. At the SLS all interruptions of the beam, of the top-up or of the orbit feedback are convoluted into a Mean-Time-Between-Distortions. It would be very interesting for the SLS to compare their performance here to other facilities. With the currently published failure data, this is not possible. If other facilities would publish failure data according to the common operation metrics, then these numbers could be used to calculate a Mean-Time-Between-Distortions for all those facilities and compare it to the SLS.
The important aspect here is, that every derived statistics would be calculated in an identical way for all facilities, thus allowing a meaningful comparison of light sources.