Apple iWork '09 - Manual (Page 198)

If  sum-values is omitted, test-values would normally contain only number or duration
values.
Examples
Given the following table:
=SUMIF(A1:A8, “<5”) returns 10.
=SUMIF(A1:A8, “<5”, B1:B8) returns 100.
=SUMIF(D1:F3, “=c”, D5:F7) returns 27.
=SUMIF(B1:D1, 1) or SUMIF(B1:D1, SUM(1)) both total all the occurrences of 1 in the range.
Related Topics
For related functions and additional information, see:
“AVERAGEIF” on page 233
“AVERAGEIFS” on page 234
“COUNTIF” on page 247
“COUNTIFS” on page 248
“SUMIFS” on page 198
“Specifying Conditions and Using Wildcards” on page 360
“Listing of Numeric Functions” on page 167
“Value Types” on page 36
“The Elements of Formulas” on page 15
“Using the Keyboard and Mouse to Create and Edit Formulas” on page 26
“Pasting from Examples in Help” on page 41
SUMIFS
The SUMIFS function returns the sum of the cells in a collection where the test values
meet the given conditions.
198 Chapter 8 Numeric Functions