Not as easy as it ought to be. I think I need to do a few things: 1. check and make sure there are no more of those !@#$!@#$#@% different sized array subtractions/multiplications. 'weight' and 'best_astro_model' need to have the same size & shape in mem_iter_pc 2. I guess just check and make sure stuff works. The weighted mean I'm using appears to be right: sum(weight * value) / sum(weight) I hate making lists that end up being two items....