What is MTTI, and how does it differ from MTTD and MTTR?

MTTI is mean time to identify: the average elapsed time from an incident being detected to the moment its cause is identified. It sits between MTTD, mean time to detect, which measures how long a fault runs before anyone notices, and MTTR, mean time to resolve or restore, which measures how long until service is back. The three are sequential: detect, identify, resolve. Definitions vary across vendors, and some use MTTI to mean time to investigate, so state your definition when you report it. MTTI is the metric most improved by better root cause capability, and it is distinct from time to RCA, which measures the written document.