We do a lot of experiments to test assumptions and build new knowledge about how the Amazon ranking algorithm works. For each experiment, we create new products that are all exactly similar except for the one characteristic we want to test. We then analyze the search results to see if the difference in the one characteristic has an impact on ranking. With the following experiment, we wanted to find out if it helps to repeat keywords in your Amazon product listings, i.e. if a higher keyword density can improve your rankings.