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X-20 Consider the following formula: ? s/yx This formula represents O a calculated correlation coefficient. O a calculated t-statistic. O a calculated standard deviation. O a calculated Z-statistic. Stationarity refers to O the size of the RMSE of a forecasting model. O lack of trend in a given time series. . O a method of forecast optimization. None of the options are correct. O the size of variances of the model's estimates. The correlograms produced by ForecastX O can be used to run a hypothesis test. O are produced only when using cross-sectional data. O include the autocorrelation plot and the partial autocorrelation plot. O should not be used with data sets containing more than 100 observations