Sorry your query has gone so long without a response.

I believe your issue is because of the square brackets you're using in the calculation. I don't think Excel likes these.

The way to tell is to copy and paste your calculation straight into a cell in the workbook and see what the error message is - errors are suppressed when using a Function and all you get is "#NAME?" as a result if there is an error in the function.

Try using round brackets (e.g.):
PS = (a * x) + ((1 - a) * (1 - z))

Further to this, you also need to put in your multiplication signs in the PD function as Excel does not use the notation xy meaning x * y.
PD = (a * (1 - x)) + (z * (1 - a))

I hope this helps.