Yet as far as Channel 4 is concerned, Ahmadinejad is still an unknown quantity for most Britons.

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But in some key repertoire for the Philharmonic, Bruckner and Schumann for instance, he is an unknown quantity.

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At 18, Prince Hamzeh is an unknown quantity (just as King Hussein was when he became king, aged 17).

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"He's seen as an unknown quantity, and VCs love for things to be very much in control, " says Facebook cofounder Dustin Moskovitz.

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But the risks of choosing such an unknown quantity are enormous.

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Mr Hollande is something of an unknown quantity in Washington, although the White House sent a team to Paris last week to get to know his closest advisers.

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Since his apparent conversion to more socially liberal policies, Mr Portillo is also a bit of an unknown quantity and has yet to map out any detailed policies.

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By comparison, his Conservative challenger, Stephen Harper, is an unknown quantity, untested by previous high office and until recently written off as a not especially competent leader of the opposition.

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The judge in the case, Dame Elizabeth Butler Sloss, will have to decide whether it is ethically right for patients who cannot freely give consent themselves to be subjected to such an "unknown quantity" - even if there is a chance it may help them.

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