A detailed NLTE investigation of Fe I lines is carried out within the framework of the MACKKL model of the quiet solar atmosphere. The authors used a realistic iron atom model which allows for the fine term structure and has about 250 levels; it involves nearly 500 UV, optical, and IR bound-bound and bound-free transitions and includes the conditions near the Fe I continuum. Errors that arise in the heights of formation, source functions, equivalent widths, and central depths of the Fe I lines within the LTE approximation are statistically studied. The authors demonstrate that the behavior of the errors can be described by a parametric set of curves depending on height of line formation, the parameter being the excitation potential of the lower level.