Effective CISSP Questions

In a risk management workshop, a team member identified a risk related to the uninterruptible power supply (UPS). If the UPS batteries are not replaced regularly, the servers may encounter unexpected power interruption as the batteries age and impact the availability. To take preventive action to mitigate this risk, which of the following should be considered most?
A. Mean Time Between Failure (MTBF)
B. Mean Time To Failure (MTTF)
C. Mean Time To Repair (MTTR)
D. Maximum Tolerable Downtime (MTD)

Kindly be reminded that the suggested answer is for your reference only. It doesn’t matter whether you have the right or wrong answer. What really matters is your reasoning process and justifications.

My suggested answer is B. Mean Time To Failure (MTTF).

Maintenance for Availability

It is not uncommon that Mean Time To Failure (MTTF) and Mean Time Between Failure (MTBF) are used interchangeably. However, MTTF applies to non-repairable items, while MTBF applies to repairable items.

Aged UPS batteries are typically replaced instead of being repaired. So, MTTF is better than MTBF in this case.


Mean time to failure (MTTF) is a maintenance metric that measures the average amount of time a non-repairable asset operates before it fails. Because MTTF is relevant only for assets and equipment that cannot or should not be repaired, MTTF can also be thought of as the average lifespan of an asset.


Mean time to failure sounds a lot like mean time between failure (MTBF), but they’re not the same.

The key difference is the type of asset used in the calculation. Where MTTF uses non-repairable assets while MTBF deals with assets that are repairable—when they break down, they can be easily repaired without spending too much.

Source: fiixsoftware

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