Research assistants, engineers, and scientists conduct research under the direction of a principal investigator; postdoctoral researchers conduct research under the direction of a faculty member for the purpose of acquiring research training; research associates conduct research with only general guidance. Positions are limited duration, funding contingent post-doctoral or research appointments that are part-time or short-term, with contracts not to exceed one year, with the possibility of renewal up to a total of three years depending on program needs, funding, and performance.