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Computers are extremely efficient at crawling, storing and processing huge volumes of structured data. They are great at exploiting link structures to generate valuable knowledge. Yet there are plenty of data processing tasks that are difficult today. Labeling sentiment, moderating images, and mining structured content from the web are still too hard for computers. Automated techniques can get us a long way in some of those, but human inteligence is required when an accurate decision is ultimately important. In many cases that decision is easy for people and can be made quickly - in a few seconds to few minutes.
Pelt et al. (Sun,) studied this question.