Say I have a model that predicts hair color in a population: $X_1\%$ is brown, $X_2\%$ is black, $X_3\%$ is blonde, and in general color $i$ occurs $X_i \%$ of the time. Now, I go out and measure the population and find $Y_1\%$ brown, $Y_2\%$ is black, $Y_3\%$ is blonde, and in general color $i$ occurs $Y_i \%$ of the time (let's assume this comes from looking at $N$ people). All of my $X_i$'s and $Y_i$'s sum to $100\%$.
What would be the proper statistical measure to see if my model accurately predicted the percentage breakdown of hair color?
Something else you can do is to compute the MRSE (Mean Root Square Error)
– Abdoul Haki Jun 10 '19 at 14:44