A statistical effect caused by random errors in the observed parallaxes of stars which leads to an underestimation of the stars’ distances and hence a corresponding error in their derived luminosities. It occurs because the number of stars increases with distance from Earth, and hence more stars in any given sample are likely to have their distances underestimated due to observational error than overestimated. The effect was first calibrated in 1973 by the American astronomer Thomas Edward Lutz (1940–95) and the American-Canadian statistician Douglas Herson Kelker (1940–2017).