The Alcock-Paczyński effect (AP effect) is an illusion affecting the determination of the depth of galaxy clusters: if you determine its depth through the relative redshifts of the individual galaxies using a wrong Hubble parameter, the cluster will appear flattened or elongated along the line-of-sight. This would occur even if there is no redshift space distortion (RSD, the effect of peculiar velocity of individual galaxies within the cluster on the apparent depth), e.g., if each galaxy were frozen in place within the Hubble flow. A redshift survey interpreted by a particular cosmological model's calculated Hubble parameter might reveal this distortion statistically if RSD's can be compensated for yet the implied averaged shape of the galaxy clusters is not spherical, which should be the shape of the average of a sufficient number of clusters. This is termed the Alcock-Paczyński test (AP test) of the cosmological model, providing a means to calibrate it or to eliminate it if its best calibration still yields distortions growing or shrinking with redshift. Such tests have been tried, but effective accommodation of the RSDs proves to be a substantial challenge to producing convincing results.