Severe problems arise for the comparision of theory with observational data.
Observational data refer, of course, to the real inhomogeneous universe. In order to derive from these data the global parameters of cosmological models, one has to apply corrections to account for local effects. This procedure is perhaps not yet well understood. A certain area on a photographic plate is taken, representing a certain solid angle in the sky, and attention is fixed on a number of small nebulous patches and their spectra. For each patch its Doppler shift s and apparent brightness b are measured, and the patches are counted.