20. Verification of Parallaxes

Hipparcos parallaxes will play a major role in the astrophysical applications of the Hipparcos results and in this respect their accuracy is more important than their precision, at least for all the investigations of statistical character. In this Chapter, the systematic errors of the Hipparcos astrometric parameters, including the parallaxes, are evaluated by examining the possible sources of bias arising in the data reduction process. Then, the external errors of the parallaxes are further studied on the basis of individual or statistical comparisons to ground-based distances. The validity of the Hipparcos standard errors are investigated as well.