Once a structure solution has been achieved, either by Heavy Atom Methods or Direct Methods, there are actually two structure models, a calculated model based on the approximate co-ordinates obtained from interpretation of a three dimensional electron density map:

The F tables list 10F

The task now is to adjust the various atomic parameters so that the calculated structure factors match the observed structure factors as closely as possible. One of the ways to measure the agreement between the observed and calculated models is with the Residual Index (more commonly referred to as the R-factor):

(At the present time, most small molecule crystallographers refine on F