Enter 0.5, 1, S, and HS in separate cells. I'll use J1:J4 for this example. You can hide the column you enter them in if you don't want your users to see them.
Select the cells you want to apply data validation to. Select Validation from the Data menu. On the Settings tab, under Validation Criteria, select Allow > List. Where it says Source, enter =$J$1:$J$4 or just collapse the RefEdit box and select the range that contians 0.5, 1, S and HS.
On the Error Alert tab, make sure "Show Error Alert..." is checked. Also, make sure you have the "Stop" style selected, or else the user will only get a warning when he enters invalid data.
Press OK and you're done. If you want to make sure that the user can't delete values in the cells with Data Validation, lock them and protect the worksheet.
-Ben
Ben, that's great. Thank you very much.
Kind Regards
Andy H