A general technique of frequentist prediction intervals is to find and compute a pivotal quantity of the observables "X", ..., "X", "X" – 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" 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.