A method and apparatus for correcting video data signals for addressing an active matrix electroluminescent display device wherein input video signals are modified in accordance with stored electrical characteristic parameter values for each drive transistor (20) employed to control the current through a respective display element (11). The stored values are continually updated to ensure accurate data signal correction which counteracts variations in the electrical characteristics of each drive transistor such as threshold voltage drift, for example. A power line (10) supplies current to n display elements (11). Thus, n sets of data relating to the current through the power line are collected during normal operation of the display, for example. The data is used to calculate updated characteristic parameter values for each drive transistor (20).