Slope, IF, Multiple Data Points

SLOPE() ignores boolean, text, and other non-numeric data, but it will include 0 values in the regression. These kind of array functions work by replacing the "values to be ignored" with some kind of non-numeric data in the input array (usually FALSE or empty string ""). The first thing I did to debug is to use the evaluate formula tool to see what the IF() array functions are returning and see if it looks right. https://support.office.com/en-us/art...6-a70aa409b8a7 Follow along in your own copy of Excel.

1) Select D8 and call up the evaluate formula tool
2) Step through each step of the function until it evaluates the first IF() array.
3) Note the results of the array (fortunately, your arrays are small enough to fit in the small window MSFT provides). We are expecting a list of 5 numbers followed by FALSE or something. When I run it, I get a bunch of 0's. As noted, 0 values are included in the regression.
4) The problem is that the IF() is returning 0 for the value_if_false argument of the IF() functions instead of something else. The solution, then, is to fix the IF() functions so they return a non-numeric value that SLOPE() can ignore.
4a) I note that most IF() arrays do not include the comma preceding the value_if_false argument, but yours do. Try removing the extra comma to see if that causes the IF() function to return FALSE. IF($A$8:$A$20=$A8,$C$8:$C$20)
4b) Many times I don't like to leave an optional argument blank if there is any question in my mind that Excel might default to something I don't want. Try explicitly entering something into the value_if_false argument IF($A$8:$A$20=$A8,$C$8:$C$20,"ignore")

Hopefully that not only helps you resolve this problem, but helps you see how to debug and find these same kinds of problems in the future.