Single Dash : It is used to specify consecutively numbered variables. A1-A3 implies A1, A2 and A3.
Double Dash : It is used to specify variables based on the order of the variables as they appear in the file,regardless of the name of the variable. A1--A3 implies all the variables from A1 to A3 in the order they appear in the data set.
Example : The order of variables in a data set : ID Name A1 A2 C1 A3
So using A1-A3 would return A1 A2 A3. A1--A3 would return A1 A2 C1 A3.
