"There are a lot of candidates who get completely run down, and they have to take a week off - and that's the kiss of death, " says Richard Ben Cramer, author of What it Takes: The Way to the White House.

BBC: How much stamina does it take to become US president?

The book's final chapter, however, takes on a more regretful tone as the author turns his attention to climate change and what it means for cold places.

ECONOMIST: Low temperatures

One positive step, which Google already takes, is to take the author of an article into account.

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It takes a case of supreme arrogance to think that you can do better than the author of a book who spent a large chunk of his life writing and revising the story and go ahead and invent scenes and dialogue and alter events in the manner in which Peter Jackson has chosen to do.

BBC: The Two Towers: Your views