Abstract: In a nondegenerate three-level ladder-type atomic system, the propagation spectrum of a probe field through a thermal atomic vapor, theoretically and experimentally, is investigated by tuning the probe field intensity. An expression for the probe response is derived analytically using the dressed perturbation method, which predicts the presence of electromagnetically induced absorption (EIA) when the probe field is no longer weak enough. Experimentally, the D₂ line of Rb in a room temperature vapor cell is used, the result shows that the conversion of electromagnetically induced transparency (EIT) into EIA at different probe detuning, not limited to one photon resonance condition, is achieved when switching from weak probe field to strong probe field. The main reason for the formation of EIA is the constructive interference between secondary dressed states generated by strong probe and coupling fields, which is analyzed using the dressed-state picture. It introduces a new ability to control the sign of the dispersion because many of the applications of EIT and EIA rely on the anomalous dispersion near the resonance.