Some embodiments of the present invention provide methods of operating nonvolatile memory devices. Reference data is stored in a plurality of memory cells. The reference data is read, and a threshold voltage distribution of the plurality of memory cells is determined responsive to reading the reference data. A read voltage of the nonvolatile memory device is modified based on the determined threshold voltage distribution. The nonvolatile memory device may include a main region configured to stored data and a dummy region configured to store the reference data, and the methods may further include reading data from the main region using the modified read voltage.