Lutz-Kelker bias

The Lutz-Kelker bias is a measurement bias applicable to parallax measurements of numerous stars such that parallax errors are more likely to produce too short distances than too long. Given the parallax distance-measurement of a particular star, more stars on average would be within the shell-shaped region surrounding Earth just beyond that determined distance than within the similar (equally thick) shell-shaped region just short of that distance because the more distant shell has a larger radius and more volume. Thus there are more stars just beyond the measured distance which could erroneously produce this measurement than there are stars just within this distance that would do so.

The bias has generated discussion on various points, one such point being the method proposed by the authors to compensate, and another being whether and when it has any significance. Some interest regarding the bias has grown with the advent of Hipparcos and Gaia, which carry out huge numbers of parallax distance determinations, and overall, their data is likely to be affected.