Description of the illustration sumfirst.gif
Sum Method Followed by Maximum Method: This diagram shows the following calculations in a grid format:
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                  Row 1: 4+4+na=8 
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                  Row 2: 1+8+6=15 
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                  Row 3: 2+3+7=12 
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                  Column 1: Maximum of {4, 1, 2} is 4 
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                  Column 2: Maximum of {4, 8, 3} is 8 
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                  Column 3: Maximum of {na, 6, 7} is 7 
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                  Column 4 (aggregates from SUM): Maximum of {8, 15, 12} is 15