Bringing trustworthy real-world data into a blockchain in a decentralized manner is a known and important challenge that is being worked on by several projects like Augur. The prevailing answer is to incentivise humans to insert it correctly by appropriately aligning rewards for doing so, and making it difficult not to do so (schelling points). That's what the Augur project is really about, the prediction market they are building is just the first application of that system.
