Decreased time for incident resolution is the best indicator that an incident management process is effective.
Incident management is a process that aims to restore normal service operation as quickly as possible after an incident, which is an unplanned interruption or reduction in quality of an IT service. Decreased time for incident resolution means that the incident management process is able to identify, analyze, respond to, and resolve incidents efficiently and effectively. The other indicatorsdo not necessarily reflect the effectiveness of the incident management process, as they may depend on other factors such as the nature, frequency, and severity of incidents. References: CISA Review Manual, 27th Edition, page 372