Where the clock starts
It is the decision that changes the result the most, and it is rarely made. When the equipment fails? When someone reports it? When the technician arrives? An MTTR of 2 hours and one of 9 can describe the same service with a different starting point.
The honest approach is to measure the whole time, from failure to restart, and break it down: detection, travel, waiting for parts and repair. That is where the real problem shows up, and it is often waiting for materials rather than working time.
MTTR and MTBF
MTBF tells you how often equipment fails; MTTR, how long it takes to come back. Together they give you availability.
And MTTA
MTTA (mean time to acknowledge) measures what happens before the repair: from the moment a request comes in until someone picks it up. If both MTTR and MTTA are high, the problem is not the repair but how requests come in and get assigned.
The usual mistake
Using MTTR to measure technicians. If most of the time is spent waiting for parts, what needs fixing is the storeroom, not the pace of work. Review your critical spare parts before you look at people.