Calcuate difference in one column based on value change in another column

Ah, then I'll have to pass, I'm afraid. Sometimes only an actual Excel file will do!

Hopefully someone else will pick up on this thread shortly. If not, you can always "bump" it after a certain amount of time has passed without you having received any responses.

Regards

Sorry... I didn't realize it mattered. The spreadsheet
file is now attached.

I'll paste what I asked above:

Okay going back to your explanation here, I understand
how anything that isn't a natural number gets reduced
to #DIV/0 (error) and therefore left out of the array.
It will therefore use the row corresponding to the
number of 1's (my columns are either 0 or 1).

So let's look at my attached spreadsheet that was
calculated correctly in AI63 as

=IF(AND(COUNTIF(AH62,0),AH63=1),LOOKUP(1,0/AH$2:AH62,$B$3:$B3940)-$B63,"")

So let's see... it should have done B61-B63 (it did).

But how did it get B61? AH2:AH56 are all blank so
those are ignored, right? AH57:AH58 are 0s, which
become errors upon reciprocation so those are
ignored. AH59:AH60 are both 1s and then you have
the 0 in AH61. So if it is looking at the array
from AH2:AH62 then wouldn't that reduce to just
two cells: AH2 and AH3? Then it if took the last
one, AH3, wouldn't it look at the corresponding
second piece of data which would be B4 (since
it starts at B3)? B4 would be wrong.

Thanks,
Mark