1. A level placement method for q-level cells of solid-state memory, where q>2, the method comprising: controlling reading of a group cells, programmed to respective levels of a predetermined plurality of programming levels, at a series of time instants to obtain a sequence of read metric values for that cell; processing the sequence of read metric values for the group of cells to derive statistical data as a function of time for each level; processing said statistical data to determine at least one parameter of a model defining variation with time of the statistical data; and calculating, using a processor, from a variation of said parameter as a function of level and said model, a set of q programming levels which has a pre-determined property over time.