Mr Sides's findings may also reflect a phenomenon identified by Moody's Analytics: over time, states' economic performance has converged, and the variance between their growth rates now matches the lowest in 70 years.

ECONOMIST: The economy

The variance in comparative values between the two metals makes it difficult to pinpoint a magic ratio that suggests one metal is cheap or expensive relative to the other.

FORBES: Silver's Bubble And Volatility

Its called a liquidity premium and its variance over time drives both difference in volatility and average return between investment vehicles.

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