Malmquist Bias

The Malmquist bias refers to an effect in observational astronomy which leads to the preferential detection of intrinsically bright objects. It was first popularized in 1922 by Swedish astronomer Gunnar Malmquist (1893–1982), who then greatly elaborated upon this work in 1925. In statistics, this bias is referred to as a selection bias and affects the survey results in a brightness (or equivalently, apparent magnitude) limited survey (referred to as being magnitude-limited), where stars below a certain apparent brightness are not included. Since observed objects (stars, galaxies, etc.) appear dimmer when farther away, the brightness that is measured will fall off quickly with distance until their brightness falls below the observational threshold. Objects which are more luminous, or intrinsically brighter, can be observed at a greater distance, creating a false trend of increasing average luminosity (intrinsic brightness), and other related quantities, with distance. This effect has led to many spurious claims in the field of astronomy. Properly correcting for these effects has become an area of great focus.