Data validation

Hi,

I would like to create a conditional data validation in a colum. Each cel, f.ex. B1 looks to the value in cel directly to it's left (in this case A1), and based on that value will select a range of values as list of validated values for the actual input in cel B1.

Does anyone now wether this is possible and if so, can you explain to me how?

Thank you for your help!!!

Peter