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It is recommended that you create a separate user profile model for app-specific functionality that relates to a User of your app. It’s best to create a UserProfile model which has a OneToOneField link to User object. Then, your Stat model can ForeignKey to UserProfile instead of User. This is to provide abstraction to the User object and give flexibility if you decide to create another Django app in the same project that requires different specifications for the user. Also, with UserProfile you can add customised fields that are app specific.
Another thing you can do to improve this modelling is create separate entities for each stat. For example, HeightWeightStat, WaistStat. But, this is not completely necessary and totally depends on your preference.
To display all of the user related Stat instances in the admin page, use Django Admin inlines: https://docs.djangoproject.com/en/1.10/ref/contrib/admin/#inlinemodeladmin-objects