1. Go to Grades, Run calculations
2. Open the saved calculation parameter set
3. Open the Course Average calculation and remove all lines except for Tri 1 and leave a factor of 1

Or

Unmark the grade of NG to  be included in Course Average calculations
1. Go to Configuration, Registrar Setup, Translation tables
2. Open the Translation Table defined on the course record
3. In Grade view,  the grade of NG to be included in Course Average Calculations
4. Save and close. Click yes when prompted that grades will be updated
5. Run the Course Average Calculation