Problem with Data Validation

The only problem with data validation is that because it's invisible to spreadsheet users, many times the only way they know they've hit a cell that uses it is when they try to make what the cell now considers an invalid entry. When anybody tries to complete an entry with text or a value that is verboten in the cell, Excel beeps at them and then displays an alert dialog box with the following unhelpful message:

The value you entered is not valid.
A user has restricted the value that can be entered into this cell.

The user can then click the Retry button and try to put something in the cell that Excel does find acceptable, or he or she can click the Cancel button and forget the whole thing.

To give users a fighting chance at rectifying their error by making an acceptable entry in the cell, I heavily suggest that you assign an input message to the cell explaining to the poor user what kind of entry is now valid. To do this, open the Data Validation dialog box (Data → Validation) and then click the Input Message tab.

There, you can enter a title for the message in the Title text box (this appears in bold at the top of the text box displaying the input message) as well as the explanatory message text you want displayed in the Input Message list box.