Patent ID: 8873285

Claim:
A method for operating non-volatile storage that includes a plurality of NAND strings of non-volatile storage elements, the method comprising: sensing threshold voltages of selected non-volatile storage elements associated with the NAND strings at a midpoint of a window (W/2); biasing a first group of bit lines associated with the NAND strings at a first voltage; biasing a second group of bit lines associated with the NAND strings at a second voltage that is different from the first, the first group of bit lines correspond to ones for which a selected non-volatile storage device had a threshold voltage below W/2 when sensing at the midpoint, the second group of bit lines correspond to ones for which a selected non-volatile storage device had a threshold voltage above W/2 when sensing at the midpoint; applying a read compare voltage to a selected word line associated with the NAND strings while biasing the first group of bit lines and the second group of bit lines; determining whether threshold voltages of selected non-volatile storage elements associated with the first group of bit lines are above or below a threshold voltage associated with a point one quarter of the way into the window (W/4) in response to applying the read compare voltage; and determining whether threshold voltages of selected non-volatile storage elements associated with the second group of bit lines are above or below a threshold voltage associated with a point three quarters of the way into the window (3W/4) in response to applying the read compare voltage.