A police department concerned with levels of white collar crime is interested in the number of years the typical offender works for a company while engaged in illegal behaviors. The department has hired a researcher to assess the relationship between years worked and involvement in white collar crime. Years of research have yielded a population standard deviation of 20. The consultant is given a sample of 225 convicted criminals to compare to the population and calculates a sample mean of 30 years what is the standard means?