I am interested in using the Course participation report (Course administration > Reports > Course participation) to see who has posted to various forums in a course. However, it seems like checking the Activity completion box for the forum counts as 1 post. Unchecking the box counts as another post, etc.
As a way to test:
1. Create a forum with Activity completion enabled ("Students can manually mark the activity as completed").
2. Enroll a student in that course, and log in as that student.
3. As the student, place a check in the completion box for that forum.
4. As the teacher (or some admin), run the Course participation report, making sure to select "Show actions = Post" and "Show only = Student", and selecting other appropriate parameters for your forum, time period, etc.
5. Observe that the student has "Post = Yes (1)", even though they did not write a post in that forum. I would expect "Post = No".
I think this happens because the report sees that an event with the appropriate crud value is recorded in the log, for the matching userid and contextinstanceid. Setting Show Action = Posts looks for crud = create, update, or delete. And the Activity completion event is recorded as a Update, with values that match the userid and contextinstanceid.
The following Tracker issues seem mildly related, but not explicitly about Activity completion.