Top: angular correlation function measured from the VIPERS W1 mock samples. In order to enhance the signal-to-noise ratio, we show only the mean over 153 realisations. The angular correlation function of the parent and selected sample are shown with a cyan and magenta line, respectively. The two dashed vertical lines indicate the typical angular size of the slits and the raw spectra. Bottom: the completeness function, extracted from the mean of the 153 W1 mock samples (magenta line), is shown. The corresponding quantity measured from the VIPERS dataset is shown with red circles.