A memory device has a memory cell array with memory cells. A page buffer group generates page buffer signals according to a verify read result of the memory cells. A page buffer decoding unit generates a decoder output signal corresponding to the number of fail bits from the page buffer signals based on a first reference current. A slow bit counter outputs a count result corresponding to the number of fail bits from the decoder output signal based on a second reference current corresponding to M times the first reference current, where M is a positive integer. A pass/fail checking unit determines a program outcome with respect to the memory cells based on the count result and outputs a pass signal or a fail signal based on the determined program outcome.