In addition to Bob's site, which many consider the "bible" of the *users*
evolution of SumProduct,
as opposed to Redmond's original intention for the function,
here's my 2¢ from an old post of mine:
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The determining factor as to what form to use should be by how the
calculating data range(s) are to be populated.
The main difference between the unary form and the asterisk form is that
when using the asterisk, *all* the data in the calculating range(s) *must*
be numeric.
That numeric data can even be numeric text - just as long as it looks like a
number, the asterisk form will calculate it.
If it's not in any numeric form, it will return an error.
The unary form will simply by-pass any text or non-numeric data, and
complete the calculation using only *real XL recognized* numbers.
Therefore, if you're populating your calculating ranges with formulas that
may return nulls ( "" ), or text messages (such as "No Data Present"), then
the unary form is the *only* way to go.
However, if data is to be either keyed in, or imported, the asterisk form
*should* be the form of choice, since it will calculate the various forms of
numbers that are usually imported from other apps or the web.
Imported numeric text will by-passed with the unary form, without any
indication as to what data was or was not used in the calculation, which may
produce inaccurate returns.
Another difference between the two forms, is that the asterisk form *must*
be used when *uneven* range sizes are incorporated in calculating 2
dimensional (row v. column) formulas.
With all things being equal, the unary form is supposedly the faster to
calculate, making it the choice for very large ranges.
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