It is determined by their pay grade and their length of time in military service. Basic Pay, also known as Base Pay, is the same for all the services. The enlisted pay chart includes pay rates for all grades from E-1 through E-10 and for durations of military service ranging from less than four months to over 40 years.
Perturbation is an anxiety or disturbance, or is a cause of disturbance, or a from what is normal, especially when the deviation is caused by an outside influence.
In technical terms, QED can be described as a perturbation theory of the electromagnetic quantum vacuum. Richard Feynman called it the jewel of physics for its extremely accurate predictions of quantities like the anomalous magnetic moment of the electron and the Lamb shift of the energy levels of hydrogen.