What does the _ALL_ keyword do in SAS?

The ALL keyword in SAS is used to reference all variables in a dataset when performing operations such as summation. When you use ALL in a procedure like PROC MEANS or when creating a new variable in a DATA step, it tells SAS to consider every variable in the dataset for the specified operation.

For instance, if you want to compute the total of all numerical variables in a dataset, you can effectively use a statement like SUM(_ALL_). This ensures that all variables, regardless of their names, are included in the computation, which is particularly useful when you don't want to manually list each variable.

While the other options suggest related operations, they do not accurately capture the function of the ALL keyword. For example, summing character variables or summarizing variables generally refers to specific actions that may not apply to every variable type or could require more explicit referencing. Thus, the clarity offered by using ALL is significant for simplifying the programming process and ensuring comprehensive analysis of the dataset.