Techniques for early detection of degradation in NAND Flash memories by measuring the dispersion of the threshold voltages (VT's), of a set (e.g. page) of NAND Flash memory cells during read operations are described. In an embodiment of the invention the time-to-completion (TTC) values for the read operation for the memory cells are used as a proxy for dispersion of the threshold voltages (VT's). A Dispersion Analyzer determines the dispersion of the set of TTC values. In one embodiment the delta between the maximum and minimum TTC values is used as the dispersion measurement. If the measured TTC dispersion differs by more than a selected amount from a reference dispersion value, a warning signal is provided to indicate that the page of memory has degraded. The warning signal can be used to take appropriate action such as moving the data to a new page.