There are times when you want to modify the observations in a SAS data set without replacing the data set. You can do this in a DATA step with the MODIFY statement. Using the MODIFY statement, you can replace, delete, or append observations in an existing data set without creating an additional copy of the data. In this chapter, you learn to modify all the observations in a data set, match observations using a BY statement, and locate observations using an index.

When you modify data, it is often essential to safeguard your data and track the changes that are made. In this chapter you learn how to create integrity constraints to protect your data. You also learn two different methods of tracking changes, audit trails and generation data sets. You use audit trails to track changes that are made to a data set in place, and you use generation data sets to track changes that are made when the data set is rebuilt.