1. An operating method of a memory system including a nonvolatile memory device, the operating method comprising: reading a plurality of first bits from an observation memory cell through a plurality of read operations using different read voltages to configure a first read data symbol; reading a plurality of second bits from a plurality of interference memory cells adjacent to the observation memory cell through a plurality of read operations using different read voltages to configure second read data symbols; and determining a logical value of the observation memory cell, based on the first read data symbol and the second read data symbols, wherein the first read data symbol is determined according to a number of the first bits and values of the first bits, the second read data symbols are determined according to a number of the second bits and values of the second bits.