The Customer Average Interruption Frequency Index (CAIFI)^[1] is a popular reliability index used in the reliability analysis of an electric power system.^[2] It is designed to show trends in customers interrupted and helps to show the number of customers affected out of the whole customer base.

${\displaystyle {\mbox{CAIFI}}={\frac {\mbox{Total Number of Customer Interruptions}}{\mbox{Number of Distinct Customers Interrupted}}}}$