A data storage device includes a nonvolatile memory device configured to store a codeword; and a controller configured to read the codeword from the nonvolatile memory device, and perform a decoding process for the codeword, wherein, when performing the decoding process, the controller calculates a flag of the codeword, calculates an expected number of errors by applying an adjustment coefficient to the flag, compares the expected number of errors to an allowed number of errors, and skips or performs a decoding operation for the codeword depending on a comparison result.