where "F" is the structure factor. A similar quality criterion is "R"free, which is calculated from a subset (~10%) of reflections that were not included in the structure refinement. Both "R" factors depend on the resolution of the data. As a rule of thumb, "R"free should be approximately the resolution in angstroms divided by 10; thus, a data-set with 2 Å resolution should yield a final "R"free ~ 0.2. Chemical bonding features such as stereochemistry, hydrogen bonding and distribution of bond lengths and angles are complementary measures of the model quality. Phase bias is a serious problem in such iterative model building. "Omit maps" are a common technique used to check for this.