Rating Formula Design (conceptual not coding)

Question

One element of my site is a rating system. I am puzzled a bit by how best to set up the formula and I hope someone with more math aptitude can help me.

Users upload pictures that are rated 1-10 by other users. The users rating then is the average of those scores.

Simple enough. However, I want to add some system which rewards users for uploading more pictures. SO that the formula would be average of ratings + some function of the number of pictures uploaded.

An example might be, Rating = AVG + .05 * Count

This formula would be somewhat fair for users who have uploaded 1-20 pictures... However, if users upload 2000 pictures they will have bipassed the entire rating system and automatically will have a 10/10.

So, my limited knowledge of post algebra math is failing. What would be some formula that would produce the desired effect? The word "log" keeps bouncing around in my head--but I honesty can't remember anything about why... :)

Answer 1

Just do something like:

avg + numofpics*scale*(.9^numofpics)

This will make it do that as they upload more pictures they get less and less. You can change .09 (the rate of decay) depending on how many pictures you expect the average user to upload.

This the equation used for half life decay