1. Navigate to Setup > App Setup > Contacts > Fields
  2. Under Contact Custom Fields & Relationships, click "New"
  3. Select "Roll-up Summary", click next
  4. Enter Field Label and Field Name, then click next
  5. For Summarized Object, select "Shift Assignments"
  6. Select Roll-up Type "Sum", and the Field to Aggregate "Credit Hours"
  7. Select "only records meeting certain criteria ..." and use the following filter: "Attended equals True", and click next
  8. Make the field visible for appropriate profiles, click next
  9. Choose page layouts, and click save