For regression problems we have a directly analogous baseline: predict the average value over the population (usually the mean or median). In some applications there are multiple simple averages that one may want to combine. For example, when evaluating recommendation systems that internally predict how many “stars” a particular customer would give to a particular movie, we have the average number of stars a movie gets across the population (how well liked it is) and the average number of stars a particular customer gives to movies (what that customer’s overall bias is).
