Lutz Kelker bias

The Lutz-Kelker bias is a systematic bias that applies to measurements of astronomical distance. In particular, it causes measured parallaxes to stars to be larger than their actual values. This causes inferred luminosities and distances to be too small, which poses a problem to astronomers trying to measure distance. The existence of this bias and the necessity of correcting for it has recently become relevant in astronomy with the precision parallax measurements made by the Hipparcos satellite.