ID Statement
ID variables ;

When one of the MODEL statement options CLI, CLM, P, R, and INFLUENCE is requested, the variables listed in the ID statement are displayed beside each observation. These variables can be used to identify each observation. If the ID statement is omitted, the observation number is used to identify the observations.

Although there are no restrictions on the length of ID variables, PROC REG might truncate ID values to 16 characters for display purposes.