Appendices A and B are only available in electronic form at http://www.edpsciences.org

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We note, that e.g. the photo-z selection for cluster galaxies at z=0.41 based on pure broad-band photometry in Kodama et al. (2001) used a 5.33 times wider redshift bin, which led to a 33 times higher estimated field contamination: the physical size of their studied area was $\sim$ $3\times$ larger as well, and the luminosity cut was marginally fainter.

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