Dr Melnick explained that to predict an earthquake's magnitude, seismologists multiply plate convergence rate by the number of years since the last great event.

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An optimal currency area would require convergence on monetary, fiscal, exchange rate, and political policies, which in turn would lead to productivity and growth rate convergence, explained Roubini.

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Paradoxically the halo effect of early interest rate convergence allowed a greater divergence in fiscal policies.

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The researchers from McGill University in Montreal noted a convergence in the rate of deaths from HIV and heart disease as contributing to the narrowing of the life expectancy gap.

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The pattern of swift, uninterrupted decline is now taken as the norm: the UN uses it to project a worldwide convergence towards the replacement rate of fertility (2.1, the rate at which a population stabilises in the long term).

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The convergence in yields reflects mainly the removal of exchange-rate risk since the creation of the single currency, but that still leaves default risk.

ECONOMIST: World economy