Meme fitnessThe overall survival rate of a meme m can be expressed as the meme fitness F(m), which measures the average number of memes at moment t divided by the average number of memes at the previous time step or "generation" t - 1. This fitness can be expressed in a simplified model as the product of the fitnesses or survival rates for each of the four stages, respectively assimilation A, retention R, expression E and transmission T: