A shot of espresso is roughly twice as concentrated by volume as drip coffee, yet a typical espresso delivers 60–110mg of caffeine, while a mug of filtered drip can carry 80–280mg. The espresso drinker is sipping something stronger — but less of it, which is what actually determines the dose. A 2017 review published in IntechOpen's The Question of Caffeine examined all the variables that make caffeine content so hard to predict. The bean variety makes a difference. Arabica — the specialty-coffee standard — tops out at about 1.5% caffeine by dry weight. Robusta, the variety that shows up in budget blends and most Italian espresso, can hit 2.4% — close to double. Roasting burns off some of it, but only 11–20% at most. A dark roast and a light roast from the same bean end up much closer than most people assume — the bitterness people associate with "strong" is from other compounds, not caffeine. One study of espresso across coffee shops found cups ranging from 51mg to 322mg — a six-fold spread.…