Combine 2 formulas to ignore errors?

Hi,

I am trying to create a sheet which had a lot of info on one sheet and then a neatly presented second sheet which calculates values (such as turnover per square metre). On the second sheet I have formulas such as =SUM(BaseData!I3/BaseData!E3) The cells which are referred to in this formula have no data in which I have put as N/A on the first sheet, therefore in the final sheet I am confronted with a #VALUE! error.

I want this to display some sort of message such as 'N/A' or 'Not Available' rather than the error and (more importantly) I need these to be ignored from any kind of further formulas such as the ranking of these values or the averaging. Having searched on Google and this forum, it appears that IFSERROR OR IFERROR could be a solution, but I'm not sure of the exact formula or how to combine it with the existing formula.....

Apologies if the wording of this post isn't very clear, I can post a picture if it would be easier to explain? I'm very new to the more advanced side of Excel and so any help would be greatly appreciated!

Thanks in advance for any help that you can offer!
Ross

Thanks, but how do I combine =IF(ISEEROR(Expression),"Not Avialable",Expression) with the existing =SUM(BaseData!I3/BaseData!E3) that is already in the cell?