You can use either the MODIFY or UPDATE statement to update a master data set with information in a transaction data set. Chapter 6 includes examples that use the UPDATE statement. Chapter 7 includes examples that use the MODIFY statement.
The MODIFY statement has many applications while the UPDATE statement is limited to updating a master data set. You can use the MODIFY statement to perform the following tasks:
process a file sequentially to apply updates in place (without a BY statement)
make changes to a master data set in place by applying transactions from a transaction data set
update the values of variables by directly accessing observations based on observation numbers
update the values of variables by ...