A general technique of frequentist prediction intervals is to find and compute a pivotal quantity of the observables "X"1, ..., "X""n", "X""n"+1 – meaning a function of observables and parameters whose probability distribution does not depend on the parameters – that can be inverted to give a probability of the future observation "X""n"+1 falling in some interval computed in terms of the observed values so far, formula_1 Such a pivotal quantity, depending only on observables, is called an ancillary statistic. The usual method of constructing pivotal quantities is to take the difference of two variables that depend on location, so that location cancels out, and then take the ratio of two variables that depend on scale, so that scale cancels out.