“The average number of work items in a stable system is equal to their average completion rate, multiplied by their average time in the system.” ~John Little, 1961

The text above is from "A Proof for the Queuing Formula" by Little,
J. D. C. (1961). It is knows as the Little’s law.

By solving this simple first equation you are able to find out the
average time for work items in your system. My whiskey bar provides us a great stable system example to illustrate how
you can apply Little’s law to track the average lead time.

**My whiskey bar**

my bar |

Whenever a bottle finishes, I remove it from the bar. Then I open a new
one, and add it to the bar. My bar is a stable system: the rate at which whiskey bottles enter the bar
is the rate at which they exit.

**Let's apply Little's law**

Little’s law: “The average number of work items in a stable
system is equal to their average completion rate, multiplied by their average
time in the system.”

Or

*The average number of work items in a stable system*

*=*

*average completion rate*

*X*

*average time in the system*

Using my bar terms:

*12 bottles (number of whiskey bottles in my bar)*

*=*

*6 bottles / year (average completion rate)*

*X*

*average time in my bar*

Therefore,
the average time a whiskey bottle stays in my bar is 2 years.

Give it a
try! Go ahead and apply the Little’s Law formula to your stable system. Similarly to
my bar example, given the average work items in the system (WIP) and the completion
rate (throughput), you can derive the average time in the system (lead-time).