The fourth category of metrics is related to the reliability and usability of your app, as well as the feedback and satisfaction of your users. To measure this, you can track metrics such as load time, crash rate, crash reports, app ratings, app reviews, and net promoter score (NPS). For instance, you can measure the average time that your app takes to launch or load a screen or page. Additionally, you can calculate the percentage of sessions that end in a crash or an error in your app. You should also document the details of the crashes or errors that occur in your app, such as the device, the OS, the app version, and the stack trace. Furthermore, you should track the average rating that users give to your app in the app store or other platforms. Finally, it's important to measure user comments or feedback that they leave for your app in the app store or other platforms. To collect these metrics, you can use tools such as Firebase, Crashlytics, App Store Connect, or Google Play Console.