@mmcdara 

Thanks for this - much appreciated. 

I am considering case 2 so I think this should do the trick!

@acer 

Thanks for the suggestion - most interesting.

Actually, I initially required a surface approximation; the interpolation is interesting ... I never considered that option but will certainly look into this option.

Thanks again ...

@Carl Love 

This is appreciated .... many thanks, Carl.

@tomleslie 

Thanks for this ....  this input and guidance is much appreciated.

When I get this resolved, I'll post my results.

@Kitonum 

Thanks for this.

@Rouben Rostamian

It appears that importing the source data from Excel requires a conversion to vector. 

Using the Assistant to import the source file, I used the routine ..

> convert (Import("c:\\ ....etc), Vector)

and 

> convert (Import("c:\\ ...etc), Matrix)

and that did the job!

Thanks again.

@Rouben Rostamian  

Thanks for the reply.

Yes ... this should output a finite sum of terms. In this case, the sum is within square brackets and so, is a list ... therein lies the problem.

Now, I have imported the source data tables from Excel, so I'm wondering if the problem lies there. The matrices appear to be ok ...

I'll battle on!

@Markiyan Hirnyk 

To be clear ...

vv originally provided the solution .... and it works.

Since then, I have tested it in several applications to my great satisfaction.

@Markiyan Hirnyk 

Markiyan,

Here's the table in question.

Thanks for pursuing this!

Z-Chart_&_Loss_Function.pdf

@Markiyan Hirnyk 

Hi Markiyan.

I like that ...!

The function in question, L(z), is the standard loss function and comprises both the density and distribution functions of the standard normal distribution.There are tables of L(z) values readily available given (for instance) z in the range [-3,3].

Given a known L value, I was seeking to extract the associated z-value and our friend has successfully supplied a routine to do just that. 

I hope that clarifies matters.

@vv

Perfect .... this addresses the problem. Nice routine.

The numerical results align to the tabulated values.

Thanks so much. 

@Markiyan Hirnyk 

Now, that's efficiency!

I'm obliged .... thanks.

@Kitonum 

I'm very grateful .... thank you!

@gkokovidis 

Many thanks, Georgios ... much appreciated!

@tomleslie 

Thank you for clearing this up .... I appreciate your help.