  The next generation of large scale surveys will not only measure cosmological
parameters within the framework of General Relativity, but will also allow for
precision tests of the framework itself. At the order of linear perturbations,
departures from the growth in the LCDM model can be quantified in terms of two
functions of time and Fourier number k. We argue that in local theories of
gravity, in the quasi-static approximation, these functions must be ratios of
polynomials in k, with the numerator of one function being equal to the
denominator of the other. Moreover, the polynomials are even and of second
degree in practically all viable models considered today. This means that,
without significant loss of generality, one can use data to constraint only
five functions of a single variable, instead of two functions of two variables.
Furthermore, since the five functions are expected to be slowly varying, one
can fit them to data in a non-parametric way with the aid of an explicit
smoothness prior. We discuss practical application of this parametrization to
forecasts and fits.
