A method comprises providing first and second semiconductor devices. Each device comprises a transistor having a split gate electrode including first and second gate portions. Each device has a respective ratio between an area of its first gate portion and a sum of areas of its first and second gate portions. For each device, a stress voltage is applied to the first gate portion, but not to the second gate portion. For each device, the first and second gate portions are biased with a common voltage, and data are collected indicating a respective degradation for each device due to the stress voltage. The degradation has a component due to time dependent dielectric breakdown (TDDB) and a component due to bias temperature instability. From the collected data extrapolation determines the degradation component due to TDDB.