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, physical fitness for the job, and signs of alcoholism or drug addiction in order to protect worker health. Interviews are also part of the assessment process.