This is really useful. I've had some success with this method but tried to extend it a little without any joy.

I am wanting to vary the criteria if cell AB6 is blank. The following is successful whether ISBLANK is true or false:

{=IF(ISBLANK($AB$6),MEDIAN($O:$O),MEDIAN(IF($A:$A=$AB$6,IF($S:$S<>"X",$O:$O))))}

I also want to check for "X" in col S for both sides of the argument, but the following formula returns 0 in error when ISBLANK = TRUE, but still functions as expected when ISBLANK = FALSE.

{=IF(ISBLANK($AB$6),MEDIAN(IF($S:$S<>"X",$O:$O)),MEDIAN(IF($A:$A=$AB$6,IF($S:$S<>"X",$O:$O))))}

Is there a limitation that stops me using an array formula for both TRUE or FALSE? Have I got something wrong in the formula maybe? Any suggestions?

It was a nice article. It helped me. Please check one thing in the section Calculate Median Excluding Zero In A Range Below there in the first line the word excluding is to be replaced by including