How to interpret the survival funnel plots

These graphs show the national average survival after specific procedures for treating congenital heart disease. The national average is shown as a horizontal grey line. Two control limits are shown; a warning limit (Green line, 98%) and an alert limit ( Red line 99.5%). Unit performances are shown as identifiable coloured symbols. If a unit's symbol is above the green line then the performance is no different from the national average. If a unit’s survival rate is below the warning limit, their performance will be closely monitored in subsequent years. If a unit’s survival rate is below the alert limit, an investigation into possible reasons and remedial actions will be launched by the appropriate professional and regulatory bodies.