The Selenium and Vitamin E Cancer Prevention Trial ( SELECT ) Eye Endpoints Study was an ancillary study of the Southwest Oncology Group-coordinated SELECT , a randomized placebo-controlled 4-arm trial of selenium and vitamin E conducted among 35,533 men , 50 years and older for African American participants and 55 years and older for all other men , at 427 participating sites in the United States , Canada , and Puerto Rico .