An apparatus for selecting a stressing voltage for compensating for changes in the threshold voltages (Vth) for drive transistors in pixel drive circuits in an active matrix OLED display having a plurality of OLED light-emitting pixels arranged in an array, comprising: each pixel drive circuit being electrically connected to a data line and a power supply line, and having a drive transistor; each drive transistor being electrically connected to its corresponding power supply line and to its corresponding OLED light-emitting pixel; each switch transistor being electrically connected to the gate electrode of its corresponding drive transistor and to its corresponding data line; first means for applying a first voltage to the power supply lines; second means for applying a second voltage to the power supply lines opposite in polarity to the first voltage; third means for producing a plurality of threshold-voltage-related signals on the data lines; fourth means responsive to the plurality of threshold-voltage-related signals for producing an average threshold-voltage-related signal; and fifth means responsive to the threshold-voltage-related signals for selecting the stressing voltage.