Thanks for the answer, this solve the main part of the problem. The only
remaining problem is that I already used validation to provide in-cell
drop-down lists for some question, so I think this is not possible to use
those 2 validation types at the same time. Is there another way to do that?
Or maybe another way to provide the drop down list? As I said in the first
question, I wouldn't like to use VB controls.

"Jim Thomlinson" wrote:

You could use custom validation. Data - Validation - Custom

In the formula add something like the cell above < "" (for example
highlight cells A2:A10 and add validation =A1<"". Be sure to uncheck ignore
blanks.

This means that the cell above the current cell must be filled in before you
can fill in the current cell. First A1 then A2 then A3...
--
HTH...

Jim Thomlinson