the *{-1,1} appears to be in the countif function
I think you want
=SUM(COUNTIF(INDIRECT("'"&{"Apr 07","May 07","Jun 07"}
&"'!F27:F48"),"<=20"})-SUM(COUNTIF(INDIRECT("'"&{"Apr 07","May 07","Jun 07"}
&"'!F27:F48"),"<18")
entered as array control-shift-enter

"chedd via OfficeKB.com" wrote:

Hi

I am using a 3D formula =SUM(COUNTIF(INDIRECT("'"&{"Apr 07","May 07","Jun 07"}
&"'!F27:F48"),({"<18","<=20"})*{-1,1})) to count ages between certain age
groups, but keep getting a 0value to where there are 8 references between the
ages of 18-20.

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