Tests are used to assess qualities and characteristics through observation and classification. To be useful, tests must be standardized, reliable, and valid. There are oral, written, and performance tests, as well as aptitude, intelligence, and personality tests administered by psychologists. Personality tests are commonly used by human resources departments. Background research checks an applicant's job history, family history, economic conditions, and recommendation letters. Medical exams check for contagious diseases, normal senses, and defects that could cause accidents to determine if a candidate is suitable for a job. Interviews are also part of the testing process.