# What is little's law

2021-05-04 14:02:59

Anyone involved in agile and Kanban can't avoid Little The laws of （Little's Law）,Little The law is an equation :

L = λ W

The variable here means ：

L = The average number of tasks or items in a queuing system over a period of time

λ= The average number of new tasks or projects entering the system within a specified time interval （ Arrival rate

= The average time spent by a task or project throughout the system

The Chinese meaning of this equation is ：

“  The number of items in the queue  ”=“  New project arrival rate  ”x“  The average time spent on a task or project  ”

First you need to know what a queuing system is ？Little Our law only applies to “ Queuing system ”, It's a queue system that has to have entry and exit . It could be a workflow system , It could be mission systems , It could be a production line .

In software development , Tasks or projects are also often referred to as user stories , Change request , Error repair, etc .

“L” Represents the number of items in the queuing system you are checking . This is also called “WIP”, Such as “ In progress ” Project , It can be almost any integer .

“λ” Represents the arrival rate and departure rate of items entering and leaving the queuing system . This is sometimes called “ throughput ” or “ Access and / Or the amount of items leaving the system ”, And sometimes expressed as λ or “A”.

Arrival rates are a bit confusing at first , But the key to remember is that it's usually just a small part . This is because you want to measure the entry of items into / Out of the system rate , Not the number of items or the time between new arrivals . therefore ,“λ” Always expressed as a fraction :

λ = ( A project ) / ( Time unit )

for example , If every new project 20 Minutes into your queue , Then your arrival rate is not 20, It is 1/20.

Last ,“W” It's the average time an item spends in a queuing system . This is also called “ Lead time ”, It can also be any unit of time . The time unit of this element needs to be the same as “λ” In the same time unit - If you measure the arrival rate in days , that “W” It will also be measured in days , And so on .

The final meaning is ：

The number of items in the system   =（ Entry and exit rates ）x（ The average time a project spends in the system ）

L = λ W  By algebraic transformation to :

W  =  L / λ

As I said before L yes WIP,  λ Can be throughput , So a variation of this formula is as follows ：

LT  =  WIP  /   throughput

LT = Average delivery time

WIP =  The average number of tasks in the queue （ Work in progress ）

throughput   = The average number of tasks that leave the system in a defined time interval

problem ： Under the assumption that throughput remains unchanged , If we keep increasing WIP, That is, the number of items in the system , Whether the delivery time keeps increasing ？

Queuing The theory points out that , As the utilization rate increases, more than 80％, The speed of the Internet has dropped dramatically （ nonlinear ）. But according to Little Law （ Given a stable system ）, If we add WIP（ Improve utilization ）, Then the time will increase linearly . Why? Little's Law There's no indication that delivery times are rising exponentially from a certain point ？

Reference resources ：Little’s Law: Isn’t It a Linear Relationship?

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