This chapter reviews the types of written documentation available to investigators, discusses preparation for examining written documentation, considers qualitative aspects of the data, and examines the application of the data to various situations. Changes in two elements of written documentation, central tendency and organizational factors, are particularly noteworthy as they may reveal much about the operator and the operator's employer. Recognizing the value of written documentation and knowing when and how to apply the information they contain to an error investigation are critical investigative skills. The quantity of information about a trait, the frequency and regularity with which the information was collected, and the reliability and validity of the measures that provide the data, affect the quality of data in written documentation. Companies generally maintain data in three types of written documentation, personnel, training, and medical records. The data in these records may provide information about the operator and his or her employer.