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There is a bit of redundancy, which users may find confusing. For example, the User column on the left displays information that is simultaneously displayed on the middle of the User Page. This real estate may be used for other means so that the information in not doubly presented. Users can find a bill from the user page, the bill listing page, or the calendar. This makes is easier for the user to find a piece of information, but the information scent used to get to it may need more fine-tuning. Overall, the ability to navigate to a page from multiple locations will help efficiency by forcing the user not to navigate through a longer route and will help learnability by not forcing the user to memorize one distinct path.
Error Prevention
This implementation is not great at dealing with error prevention. This design does not allow the user to undo transactions, although before a transaction goes through the user must review a confirmation page. In addition, information tends to be clustered together, so mis-clicks could be an issue. To deal with this, it would be a good idea to place less information on a screen.
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