Text stimulus (graph): Line graph titled 'Volunteer Participation by Age Group, National Sample.' X-axis age groups: 14-17, 18-25, 26-44, 45-64, and 65+. Y-axis percentage from 0 to 60. Plotted data: one line with points 32, 28, 35, 41, and 38. Caption says 'aggregate survey pattern across the country.' A researcher asks how national volunteer participation varies by age group. Which level is primary?