1. Check the definition of availability on Wikipedia: https://en.wikipedia.org/wiki/Availability. You will find the basic example as follow:

What is wrong with this example? What is the probability of the system being failure? Refer to the following table:

What is the quality level of this system in terms of sigma?

Basic example [edit ] If we are using equipment which has a mean time to failure (MTTF) of 81.5 years and mean time to repair (MTTR) of 1 hour: MTTF in hours =81.536524=713940 (This is a reliability parameter and often has a high level of uncertainty!) Inherent availability (Ai)=713940/(713940+1)=713940/713941=99.999860% Inherent unavailability =1/713940=0.000140% Outage due to equipment in hours per year =1/rate=1/MTTF=0.01235 hours per year. \begin{tabular}{|c|c|l|c|} \hline Sigma & ppm Defects & \multicolumn{1}{|c|}{ Yield } & Cost of Quality \\ \hline 2 & 308,537 & 69.2% & 2535% \\ \hline 3 & 66,807 & 93.3% & 2025% \\ 4 & 6,210 & 99.4% & 1218% \\ \hline 5 & 233 & 99.98% & 48% \\ 6 & 3.4 & 99.99966% & 13% \\ \hline \end{tabular}