VBA Excel function to interpolate precipitation data
Naming

Your variable's names are pretty generic. Tell me what they are - use the name to give information - the characters are free.

The same goes for the function name - IDWW. I'm not sure what it should be doing.

I imagine you'd be good with names like

firstValue, secondValue, thirdValue, firstDistance, secondDistance, thirdDistance

I don't know what those are because they aren't given a Type. Your function also doesn't have a return Type. That means you're taking in 6 variants and returning a variant. That has impact.

Variant

When you don't define or type your variable, VBA will declare it as a Variant, which are objects:


  Performance. A variable you declare with the Object type is flexible
  enough to contain a reference to any object. However, when you invoke
  a method or property on such a variable, you always incur late binding
  (at run time). To force early binding (at compile time) and better
  performance, declare the variable with a specific class name, or cast
  it to the specific data type.


Total you are working with 12 Variants and a return Variant.

You also have 6 variables made of 1 letter and 1 number. The code is already pretty abstract, but seeing something like

For i = k To j Step -1
    For n = 1 To 26
        If k * n &gt; j * n Then
            n = i * j
        Else
        n = i * k
        End If
    Next
Next


Gives me no confidence that I'll be able to decipher what's happening.

You also test like this twice -

If Value2 &lt;&gt; "" And Dist2 &lt;&gt; "" Then
If Value3 &lt;&gt; "" And Dist2 &lt;&gt; "" Then


It seems like these should be optional arguments in the function, otherwise you're forcing the user to submit nulls.

Private Function IDWW(ByVal firstValue As Long, ByVal firstDistance As Long, _
        Optional ByVal secondValue As Long, Optional ByVal secondDistance As Long, _
        Optional ByVal thirdValue As Long, Optional ByVal thirdDistance As Long) As Long


You can also give them default values. If you don't give them defaults, they choose the default value of their Type.

Optional ByVal secondValue As Long = 0, Optional ByVal secondDistance As Long = 0,


You were testing for "" which has a name = vbNullString. But what you should have been testing for is the equivalent for a Variant

If IsEmpty(var) then


Something else you aren't testing for is whether the user passes something else in the Variant, like a Range. Or maybe a negative number. These are all things that can be eliminated with the right Type.

Like here -

If Value2 &lt;&gt; "" And Dist2 &lt;&gt; "" Then
    a2 = Value2 / (Dist2) ^ 2
    b2 = 1 / (Dist2) ^ 2
Else
    a2 = 0
    b2 = 0
End If


If a2 and a3 were Long you wouldn't need to set them, they are by default 0.

At the end


'Avoid a problem if all 3 distances are empty
 If b1 + b2 + b3 = 0 Then
    IDWW = 0



Is that really what you're doing? Adding variants to get 0. But what if I submitted a negative distance? What if I submitted both a negative value and distance?