Netflix has a team of 40 freelancers who tag incoming TV shows and movies, so it’s a manual process but variable cost. From the Wired article re: Netflix:
We have more than 40 people hand-tagging TV shows and movies for us. These are typically freelancers who do this to supplement their income. All of our analysts are TV and film buffs, and many have some experience working in the entertainment industry. They obviously have personal tastes, but their job as an analyst is to be objective, and we train them to work that way.
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