Little's law states that the average number of people in a waiting line is the average customer arrival rate multiplied by the:

John Little is known for his queueing theory which is basically a theory on the probability of a customer waiting in the same line. This is applicable in every establishment that does first come, first serve basis. The probability of a person staying in line, and not changing to other lines, is expressed by the so-called Little law. It states that the average number of customers in the waiting line is equal to the average effective arrival ate multiplied with the average time that the customer spends in the waiting line. This law is very useful and valid because it does not count into factors the miscellaneous things like process distribution, service distribution, service order, etc.