  Ionization of atoms and molecules in strong laser fields is a fundamental
process in many fields of research, especially in the emerging field of
attosecond science. So far, demonstrably accurate data have only been acquired
for atomic hydrogen (H), a species that is accessible to few investigators.
Here we present measurements of the ionization yield for argon, krypton, and
xenon with percentlevel accuracy, calibrated using H, in a laser regime widely
used in attosecond science. We derive a transferrable calibration standard for
laser peak intensity, accurate to 1.3%, that is based on a simple reference
curve. In addition, our measurements provide a much-needed benchmark for
testing models of ionisation in noble-gas atoms, such as the widely employed
single-active electron approximation.
