Any simple way to do VLOOKUP combine “linear interpolation” in excel?

Question

I'm making an excel sheet for calculating z-score for infant weight/age (Input: "Baby Month Age", and "Baby weight"). To do that, I need get LMS parameters first for a specific month, from below table.

http://www.who.int/childgrowth/standards/tab_wfa_boys_p_0_5.txt

(For Integer Month number, this can be done by vlookup Method without issue.) For Non-Integer Month number, I need use some kind of "linear interpolation" approach to get an approximate LMS data.

The question is, both Trend method and Vlookup method are not working for me. For Trend method, it is not working as the raw data, like L parameters is not linear data, if I use Trend method, for the several top month, return data will far from existing data. As for Vlookup method, it just finds the closest month data.

I had to use multiple "Match" and "Index" Method to do the "linear interpolation" for myself. However, I wonder whether there is any existing function for that?

My current formula for L parameters is below:

=MOD([Month Age],1)*(INDEX('WHO BOY AGE WEIGHT'!A:D,MATCH([Month Age],'WHO BOY AGE WEIGHT'!A:A)+1,2)-INDEX('WHO BOY AGE WEIGHT'!A:D,MATCH([Month Age],'WHO BOY AGE WEIGHT'!A:A),2))+INDEX('WHO BOY AGE WEIGHT'!A:D,MATCH([Month Age],'WHO BOY AGE WEIGHT'!A:A),2)

Answer 1

If we assume that months increment always by 1 (no gap in month data), you can use something like this formula to interpolate between the two values surrounding the give non-integer value:

=(1-MOD(2.3, 1))*VLOOKUP(2.3,A:S,2)+MOD(2.3, 1)*VLOOKUP(2.3+1,A:S, 2)

Which interpolates L(2.3) from data of L(2) = .197 and L(3) = .1738 , resulting in .19004 .

You can replace 2.3 by any cell reference. You can also change the lookup column 2 for L into 3 for M , 4 for S etc.

To answer the question whether there is some direct "interpolate" function in Excel, not that I know about, although there is good artillery for statistical estimation.