As discussed earlier, when many people use the term "reliability", they are actually thinking of a characteristic that includes aspects of reliability, availability, and maintainability. In fact, these are three inter-related but separate characteristics. RAM analysis is a process that addresses all distinct aspects of these three characteristics.

• Reliability is a measure of the instantaneous likelihood that a system or device will fail in a specific period of time. For instance, if there is a 10% likelihood that a device will fail in a one-year period, the reliability for that period is 90%.

• Availability is the ratio of uptime or operating time to total time. If a device has a planned shutdown lasting one week in every ten weeks for maintenance, the availability would be 90%. If that same device also experiences an unexpected outage once during every operating cycle and that outage lasts one week, the availability would be 80%.

• Maintainability is a measure of the ability to restore the inherent reliability of a system in a ratable period of time. For examples, we can use responses you might get from your automobile mechanic. If you took your car to the mechanic with an unusual problem and he were to say, "I don't know how long it will take, but it will be reliable when I finish", the car is not maintainable. Similarly, if the mechanic were to say, "I can have it back to you in four hours, but I don't know how...