looking up and summing values

[ruberg]

I'm working with a worksheet with 4000+ rows, so I'm looking for a formula to automate a summation.

In Worksheet1 (4000+ rows) I have data from 1998 to 2008 for each month and day:

Year Mo Dy Data
1998 1 1 41
1998 1 2 35
1998 1 3 79
...
2008 12 29 39
2008 12 30 81
2008 12 31 42

In Worksheet2 (365 rows) I have data for just months and days:

Mo Dy Sum
1 1 94
1 2 103
1 3 222
...
12 29 65
12 30 211
12 31 98

I'd like to take each Jan 1 datum for each year from Worksheet1, sum them, and place the value in the cell that corresponds to Jan 1 on Worksheet2. I've primarily tried using the VLOOKUP function, but can't seem to make it work.

Any suggestions? Thank you in advance for the help!

[windknife]

Try this,
C2

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copied down.

Hope this helps,
windknife

[ruberg]

Great, that definitely worked; thank you!

Is there a simple way to take the average of all the data instead of the sum? In other words, is there another method besides the following:

=(SUMPRODUCT(--(Sheet1!$B$2:$B$5000=A2),--(Sheet1!$C$2:$C$5000=B2),Sheet1!$D$2:$D$5000))/11

And just for my own knowledge, what does "--" mean?

[windknife]

Try this,

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-- transfer True/False (Boolean) to 1/0 (Numeric)

Hope this helps,
windknife

[ruberg]

Yes, that works wonderfully.

Thanks again for your assistance!