An operating method of controller includes estimating, by a mean-bias manager, candidates of a mean bias voltage based on source read voltages corresponding to respective program states; determining, by a reliability interval manager, whether the candidates of the mean bias voltage are within confidence intervals respectively corresponding to the program states; selecting, by the reliability interval manager, the candidates of the mean bias voltage when the candidates of the mean bias voltage are within confidence intervals, respectively; deciding, by a read bias manager, target read voltages based on selected mean bias voltages or the selected candidates of the mean bias voltage; and reading, by a processor, target data according to the target read voltages.