To see the Little's law triplet, consider the line of customers at the grocery store checkout lane shown in Figure 1. Following the usual queueing theory convention, the queue includes not only the customers waiting but also the customer currently in service.
As an aside, it is useful to keep in mind that there are only three types of performance metric:
- Time $T$ (the fundamental performance metric), e.g., minutes
- Count or a number $N$ (no formal dimensions), e.g., transactions
- Rate $N/T$ (inverse time dimension), e.g., transactions per minute