This content got low rating by people.
Is a selection effect in observational astronomy and named after Gunnar Malmquist, a Swedish astronomer who first described its effects on the data.
Specifically, if a sample of objects (galaxies, quasars, stars, etc.) is limited in magnitude, then the observer will see an increase in average luminosity with distance. This happens simply because as the distance increases the weaker sources are no longer detected.
The solution to this problem is not to use a sample limited in flow or magnitude but a limited sample in volume, ie take all objects up to a given distance, making sure that the weakest objects are detectable throughout the volume.
Source: In a place in the cosmos