A schedule that contains only user groups and tests will run each test in a user group sequentially. Adding a random selector lets you repeat a series of tests in random order, thus emulating the varied actions of real users.
Think of yourself as a "random selector." You are blindfolded, and you have a bucket that contains 10 red balls and 10 green balls. You have a 50% chance of picking a red ball, and a 50% chance of picking a green ball. You pick a ball randomly—it is red. You then replace the ball in the bucket. Every time you pick a ball, you have a 50% chance of getting a red ball. Because the ball is replaced after each selection, the bucket always contains 10 red balls and 10 green balls. It is even possible (but unlikely) that you will pick a red ball every time.
Assume that a random selector contains two tests: Browse and Bid. You assign Browse a weight of 7 and Bid a weight of 3. Each time the loop is executed, Browse has a 70% chance, and Bid has a 30% chance, of being selected.
Parent topic: Representing workloads