What modification is needed to correct the SQL error in the given SAS program?

To address the SQL error in the given SAS program, the modification involving the replacement of the WHERE statement with an IF statement is crucial. In the context of SAS SQL procedure, a WHERE clause is not compatible with certain types of processing that occur on a DATA step, especially when you're trying to reference calculated columns that are not yet defined at the time of filtering.

An IF statement evaluates conditions at the DATA step level, making it suitable for filtering observations after they have been created or modified within the data step. This means that if the BMI variable is calculated later in the program, referencing it in a WHERE clause will lead to an error because the SQL environment does not yet recognize it. By switching to an IF statement, you ensure that the observation is only processed if it meets the criterion after the BMI variable has been computed.

In summary, replacing the WHERE statement with an IF statement allows for the correct sequential execution of data steps necessary for the program to function properly without generating syntax or logical errors associated with premature referencing of variables that have not yet been calculated in that context.