Which of the following metrics is NOT typically measured by Real User Monitoring (RUM)?

Real User Monitoring (RUM) is primarily focused on capturing data about users' interactions with web applications and the performance experienced from the end-user perspective. Metrics like session duration and bounce rate are essential in RUM as they provide insights into user engagement and behavior, helping to understand how long users stay on a site and how often they leave without interacting with it.

The frustration score is also relevant within RUM as it quantifies user sentiment based on various performance indicators, allowing organizations to gauge the user experience more effectively.

In contrast, backend service response time, while it is a critical part of overall application performance, is typically measured through Synthetic Monitoring or Application Performance Monitoring (APM) rather than RUM. This metric is focused more on server-side performance rather than the end-user experience, which is the primary concern of Real User Monitoring. RUM is concerned with how users perceive and interact with the application in real-time but does not involve the detailed metrics related to server responses, making backend service response time the correct answer as it is not typically measured by RUM.