A new survey of campus sexual violence in the United States has found that more than 1 in 4 undergraduate women from 33 large universities have experienced sexual assault while they were students. Fewer than 30 percent of those women, who were assaulted by force or while they were unable to consent, filed a report or sought help or counseling from their schools—typically because they believed they could handle it alone, were too embarrassed or ashamed, or felt that what they had experienced was not serious enough to merit assistance. The survey of more than 181,000 students, conducted last spring and published this week by the Association of American Universities, is the largest of its kind.