Hello! I have some concentrical spheres with many points inside. And I need to plot the total mass of points in each shell (so between 2 spheres) versus the radius of that shell (defined as (r1+r2)/2, where r1 and r2 are the radius of the 2 spheres forming the shell). I have 10 shells so my plot has 10 points and I want to fit them with a certain function and I need to calculate the likelihood associated with this fitting function. But for this, I need a sigma and I don't know what exactly would that be. So i know the mass, radius and number of points in each shell. I thought to take the error something like sqrt(number of particles) but I am not sure. What should I do to calculate the likelihood? Thank you!