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The stationary time series has a constant mean and variance and the autocovariance function depends on the interval of two time points and not depend on the points themselves. The series of deviation from the purchasing power parity is clear not stationary. It does not appear to have a constant mean at all. It probably has a downward trend. It might be stationary after removing this trend. Suppose the trend is removed, but the variance does not seem to be constant over the period....
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