How does google calculate metrics for mobile apps based on page views
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I use google analytics for my mobile, and i use `trackPageView` method to register a page view on my application. When i see in google analytics page, average visit duration is around 16 mins or for some regions it's around 32 mins+. 
I want to understand how the metrics like "Average Visit Duration" , "Visitors", "count of visits" and "visits" are calculated based on the pageViews i am sending to google analytics.