There are more possible answers here.
What we measure in experiments are called observables or variables.
By saying observable or a variable, you're talking about one aspect of one general characteristic of an object. Imagine measuring the length of a leaf. Your variable would be length, which is a characteristic of your object (leaf).
1. Stimulants
2. Hallucinogens
3. Amphetamines
4. Depressants
The smallest arteries are arterioles, the smallest vessel is capillaries, veins, and the largest is arteries.