It is a completely different approach of scheduling to make the real promises to the user. In this scheduling if there are n users logged on while you are working, you will receive 1/n of CPU time.
Similarly, on the single user system with n process running, all things being equal, each one should get 1/n of CPU time.
To make good on this promise the system must keep track of how much CPU each process has had since its creation. It then computes the amount of CPU each one is entitled to, namely the time since creation divided by n. since the amount of CPU each time each process has actually had is also known, it is straight forward to compute the ratio of actual CPU time consumed to CPU time entitled.
A ratio 0.5 means that the process had has had only half of the CPU time it should actually have had, and the ratio 2 means that the process has had twice as much as it was entitled to. The algorithm is then run to the process with the lowest ratio until its ratio has moved above its closest competitor.