              The value of sigma~a~ used in constructing the Luzzati plot for\
               the reflections treated as a test set in refinement. The\
               details of the estimation of sigma~a~ can be specified\
               in _refine_analyze.Luzzati_sigma_a_free_details.\
\
               Ref:  Luzzati, V. (1952). Acta Cryst. 5, 134-136.