The Activity Report differs from the Statistic in that it presents data based on the students. Statistics presents data based on the posts.
When you enter the Forum tool, you will be able to see the Activity Report option.
This page will list down your students and the number of threads and replies that they posted. You can search for a particular student and also specify a date range for the report to be displayed.
There is also EXPORT TO EXCEL button in this page that exports the report to an excel file.