Biases in the Main Sequence Fitting Distances to Globular Clusters based   on the Hipparcos Catalogue
  We discuss the different biases affecting the sample of field subdwarfs selected from the Hipparcos Catalogue, and used in the Main Sequence Fitting technique to derive distances to Galactic Globular Clusters. The adopted average corrections significantly affect the derived distance moduli, explaining the differences among various groups using this technique. 
Eugenio Carretta, Raffaele G. Gratton, Gisella Clementini and Flavio   Fusi Pecci