  In the standard (classic) approach, galaxy clustering measurements from
spectroscopic surveys are compressed into baryon acoustic oscillations and
redshift space distortions measurements, which in turn can be compared to
cosmological models. Recent works have shown that avoiding this intermediate
step and fitting directly the full power spectrum signal (full modelling) leads
to much tighter constraints on cosmological parameters. Here we show where this
extra information is coming from and extend the classic approach with one
additional effective parameter, such that it captures, effectively, the same
amount of information as the full modelling approach, but in a
model-independent way. We validate this new method (ShapeFit) on mock catalogs,
and compare its performance to the full modelling approach finding both to
deliver equivalent results. The ShapeFit extension of the classic approach
promotes the standard analyses at the level of full modelling ones in terms of
information content, with the advantages of i) being more model independent;
ii) offering an understanding of the origin of the extra cosmological
information; iii) allowing a robust control on the impact of observational
systematics.
