I’ve used the conditional functions for dates that Hans wrote about in post 503094 . Here’s the formula: =AVERAGE(IF((A2:A16>=F1)*(A2:A16=$F1)*($A$2:$A$16=$F1)*($A$2:$A$16<=$F2),$C$2:$C$16)). The second data series is in C.
I'd also like to do some conditional LINEST estimates, but that doesn't seem to work. I'm guessing there's a problem with the double array functions, but would appreciated any help or suggestions.