Malmquist bias

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The Malmquist bias, named after the Swedish astronomer Gunnar Malmquist (1893-1982), is a selection effect in observational astronomy. Specifically, if a sample of objects (galaxies, quasars, stars, etc.) is flux-limited, then the observer will see an increase in average luminosity with distance. This is, of course, because the less luminous sources at large distances will not be detected. The solution is then to use a sample that is not magnitude limited (for example, one that is volume limited.) Despite its obviousness, the Malmquist effect has befallen some astronomers, leading to spurious claims, such as that the Hubble constant increases with distance.