Methods and apparatus are provided for determining level-thresholds for q-level memory cells. A plurality of the memory cells are read to obtain respective read signal components. The read signal components are processed in dependence on signal level to produce a signal level vector, comprising a series of elements, indicative of the distribution of read signal components in order of signal level. The signal level vector is scanned with a sliding window of length greater than the spacing of successive window positions in the scan. At each window position, a metric Mi is calculated in dependence on the elements of the signal level vector in the window. A level-threshold for successive memory cell levels is then determined in dependence on variation of the metric over the scan.