Update StatArb.R

    # I would suggest to use use the linear regression. since there if we want 
    # the arima model to include the intercept then we have to set the 
    # 'include.mean' option to TURE in which case it will make the independence variable minus 
    # their mean (quote from function indication:  Further, if include.mean is true (the default for an ARMA model), this formula applies to X - m rather than X)

    # So I would suggest to use linear regression.