Coffee Beans, Light vs. Dark Roast

Caffeine is extremely stable during the roasting process. A change in caffeine depends on how you measure your coffee.
There is an estimated ninety bean difference between a pound of dark and light roast coffee, with the dark roast higher. During the roasting process, a bean loses its mass. The density of the bean changes; beans that are roasted longer are less dense. That is why you have more beans by mass of dark roasts.

If you measure your coffee by scoops, light roasted coffee will have more caffeine, since the beans are denser than a darker roast. However, if you weigh your scoops, darker roasts will have more caffeine, because there is less mass. Depending on how ground coffee is measured, there is only a minimal variance in caffeine content with dark and light roasts. Unlike Marijuana,  people have died from caffeine overdose.