awk average part of column if lines (specific field) match

Question

Here is a sample of my input file :

$cat NDVI-bm  
P01 031.RAW 0.516 0 0  
P01 021.RAW 0.449 0 0  
P02 045.RAW 0.418 0 0  
P03 062.RAW 0.570 0 0  
P03 064.RAW 0.         


        

Answer 1

{ total[$1] += $3; ++n[$1] }

END { for(i in total) print i, total[i] / n[i] }