A method for storing data includes designating, in a memory array including cells configured for writing a first number of bits per cell, a group of the cells to which input data are to be written at a second number of bits per cell, smaller than the first number. Dummy data that are independent of the input data are stored in a first set of one or more bits of the cells in the group. The input data are written to a second set of at least one other bit of the cells in the group.