This document discusses various metrics for measuring website availability and performance. It defines Failed Customer Interactions (FCI) as when a customer request does not receive the expected response from the application. Top Percentile (TP) measures the response time within which a certain percentage of requests are served. Real User Monitoring (RUM) measures actual page load times for real users rather than synthetic tests. RUM is important for Single Page Applications where most processing happens after page load. The document also describes using NewRelic's RUM tool to capture metrics and calculate a formula for Measured Availability.