Here's a formula I found online to add every 7th row:

=SUMPRODUCT(--(MOD(ROW(A1:A300)-1,7)=0),A1:A300)

I've modified it, of course, to fit the cells I'm working with and I
changed it to add every eighth row. (I'm actually working with cells T12 -
T428, but I made the cell range smaller for debugging purposes.)

=SUMPRODUCT(--(MOD(ROW(T412:T428)-1,8)=0),T412:T428)

It SHOULD start with T412 and add it to T420 and T428, but for some strange
reason, it adds T417 and T425 only and returns the result. Can anybody
understand why?



Now when I try the following:

=SUMPRODUCT(--(MOD(ROW(T421:T427)-1,2)=0),T421:T427)

It adds every other row just as it should (Sunday, Tuesday, Thursday,
Saturday)



This adds every third row (Sunday, Wednesday, Saturday)

=SUMPRODUCT(--(MOD(ROW(T421:T427)-1,3)=0),T421:T427)


So why does that formula work in the last two examples, but not in the
first? What I have on the spreadsheet is a row for every day of the week,
followed by a weekly total row. Then the formula should go through the
entire year of 2004 and add up the totals of each week, which is every
eighth row. The modified formula at the top SHOULD do this for the last
three weeks, but it doesn't. The totals for the last three weeks are 426,
269, and 365. But the formula returns a value of 102. That value is the
total of cells T417 (43) and T425 (59).

Why is this happening?

Thanks,
Damaeus