Since the derivation of the in-loop full-zone voltage response formula, many papers have dealt with utilizing it to work out the solution for full-zone apparent resistivity, and the conclusions have three points: unique valued, dual-valued and undefined valued. The conclusions are based on mathematical theoretical analysis. They are of course correct, but not perfect. The main characteristic of the full-zone apparent resistivity curve of observed data is continuous or discontinuous. Based on model study, the author concluded that the main characteristics of an observed data on he layered earth are as follows: the full-zone apparent resistivity curves are all discontinuous, and there are three factors, i.e., no unique value exists because measurements are discontinuous, and therefore sampling time is rarely just at the transition point; undefined value exists because observed value is greater than homogeneous half-spaces; wrongly defined value exists when the observed response is lower than homogeneous half-space's. The last term is named by the author firstly. The normalizing processing is the only method to get the continuity of the full-zone apparent resistivity.