Well, you haven't told us what data you have, what result you are getting,
or what result you expected.
One interesting point is that the range specified for your first SUM seems
to start on your Data sheet but finish on the current sheet.
You've used the Excel 2007 IFERROR function but haven't specified what to do
if the error is encountered. What happens if you remove the IFERROR and
just use
=SUM(OFFSET(Data!$J$1,2,0):OFFSET($J$1,38,0))/SUM(OFFSET(Data!$I$1,2,0):OFFSET(Data!$I$1,38,0))
[correcting the data ranges if appropriate]?
You specify the start and end points of your SUM ranges with separate OFFSET
functions, but couldn't you use OFFSET to specify the range? in other
words, couldn't you replace
SUM(OFFSET(Data!$I$1,2,0):OFFSET(Data!$I$1,38,0)) by
=SUM(OFFSET(Data!$I$1,2,0,37,1))
If you are struggling to understand what your parentheses are doing, why not
break down the formula to manageable chunks? Try the first SUM as one
formula, the second SUM as a second formula, and see whether each of those
gives you the result you expected, then you can do the divide.