Problem with if statement and empty cells

Hi there! So yeah im pretty new to VBA and I am trying for hours to use an if statement to apply a function only if the values of my column is a number (not empty). I have been using :

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But it seems that it calculates the Z-Score even if a cell is empty, resulting in a major spread (-5.85 z-score answers everywhere). I am kind of lost here, if someone has an idea of how to state it so it works I would be grateful! Thank you in advance,

Excelsior