» Calculating the Average of Every Nth Value, Excluding Zeros

Problem:

For each value of n found in column C, we want to calculate the average of every nth cell from List1. We also want to exclude any zeros from the calculation.

Solution:

Use the AVERAGE, IF, MOD, and ROW functions as shown in the following Array formula:
{=AVERAGE(IF((MOD(ROW($A$2:$A$13)-ROW($A$2)+1,C2)=0)*($A$2:$A$13<>0),$A$2:$A$13))}
Screenshot // Calculating the Average of Every Nth Value, Excluding Zeros