There are four types of variables.
Statistics provide a way of dealing with numbers.
Before leaping headlong into statistical tests, it is necessary to get some idea of how these numbers come about, what they represent, and the various forms the numbers can take. Let's begin by examining a simple experiment. Suppose an investigator has a hunch that clam juice is an effective treatment for the misery of psoriasis. He proceeds to assemble a group of patients, randomizes them to a treatment control group and gives clam juice to his treatment group and something which looks, smells and tastes like clam juice, but isn't, to his control group. After a few weeks, he measures the extent of psoriasis on the patients, perhaps estimating the percent of the body involvement, or by looking at the change size of a particular lesion.