Protective factors "modify or transform responses to adverse events so that avoid negative outcomes" and encourage the development of resilience. Their development enables students to apply them to challenges and engage in them positively that does not negatively affect their education, personal lives, or successes. Examples of these protective factors identified by Reis, Colbert and Hebert in their three-year study of economically disadvantaged and ethnically diverse students include "supportive adults, friendships with other achieving students, the opportunity to take honors and advanced classes, participation in multiple extracurricular activities both after school and during the summer, the development of a strong belief in the self, and ways to cope with the negative aspects of their school, urban and family environment." On the other hand, risk factors impede the student's ability to positively engage in their challenges and in many cases prevent these students from achieving at the same level as students who do not encounter the same situations, and can include family tragedy, having an older sibling who became involved in drugs and/or alcohol, family instability, personal pain and academic failure. "Just as risk factors and childhood stressors may co-occur within a particular population or within a particular developmental period, protective factors are also likely to occur together to some degree."