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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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.

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The point was to equalize the variance of the numbers among different eras by looking at how the all-time greats statistically stacked up to their peers during the time in which they played.

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