An operating method for a data storage device may include: determining a displacement value based on section memory cell numbers regarding a plurality of threshold voltage sections divided by a first read voltage and second read voltages; determining an adjustment direction based on the displacement value; adjusting at least one reliability value corresponding to at least one threshold voltage section among the threshold voltage sections, positioned in the adjustment direction from the first read voltage; and performing an error correction operation on data read from memory cells based on the first read voltage, using reliability values corresponding to the threshold voltage sections.