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If the sample of returns between two markets is split in half, the high return and higher volatility half will have a higher correlation than the low return half when in fact the true correlation is best measured by the whole sample correlation. This is due to the econometrics of the correlation measure and illustrates the pitfalls of estimating the correlation statistic during periods of increased market volatility. To determine whether markets really have increased their comovement during periods of increased volatility, the analyst should measure the dispersion (standard deviation) of country portfolios.