In this paper, we try to explain the differences between the Residential Variation Statistics (Estadística de Variaciones Residenciales, EVR) and the Population Census 2001 in the measurement of Spanish population mobility from 1991 to 2001. We found that the migrations captured through the Census differ noticeably from the data of the residential variations. After analysing both statistical sources in detail, we think that the Census 2001 presents important coherence problems and non-response in this variable, so that it is not a reliable statistical source with which to evaluate the current level of population mobility.

Key words: Migrations, Census, Residential Variation Statistics, Nonresponse.
JEL classification: C81, J61 y R23