Time: A Key Metric in any Value Stream


 

Since 450 BC King Solomon realized the importance of the right time for everything.  

Time is often critical to our success in daily life.

Following the right sequence of the different activities is also key.

Time is critical to any process, whether it be manufacturing, sales, distribution or business processes.


Value Stream Map: Who Does What and When

When several people are involved in a sequential process each one needs to know when he needs to do something. Synchronization is key for success.

A value stream map defines the right sequence of activities and who does what. 



Job Scheduling

In addition to defining the sequence of operations we should be able to plan when each one is involved therefore we need to know the time each job it will take. 

In a sequential process a delay in one of the steps causes a delay to all following steps so this requires a reschedule of the value stream and a quick communication to all those involved in order to react.




Time Measurement

Time is a variable we can measure with a watch, clock, or more precise instruments.

A time period, being a variable, can be characterised by it average value and standard deviation.

When variation is small, time distribution is close to a normal (Gaussian) distribution but if variation is larger it deviates becoming asymetric:

In this case the distribution is far from normal. 

The reason for this is the fact that a time period always has a physical minimum value.

On the other hand there is no maximum value: things can go wrong causing delays.  


Time Data Collection

Collecting the time when events actually happen is key to manage a process. 

Time is often automatically collected by equipment or applications. 

When reporting in a computer this measurement can automatically be provided.

Statistical process control (SPC) is possible with real time data collection


Time as an Attribute

Sometimes time is collected as an attribute: instead of measuring it we are only interested to know if a time commitment was fulfilled. 

On-Time-Delivery measures the proportion of times we delivered within the committed time. 

Obviously the information provided by this metric is very poor compared with the distribution of times seen above.

A time distribution helps us understand and take different improvement actions for the different situations. 


Time Variation

There is variation in everything we do. Even automatic events have variation. 

Statistical process control (SPC) allows us to discriminate between a significant process change and natural variation in order to react when this happens and avoid over-reaction.



Waiting Times

In a sequential process time variation in some of the steps cause an accumulation of Work-In-Process causing a bottleneck for the total value stream.

All items have to wait in front of the bottleneck and this increases the total value stream lead time.

 This also reduces the effective capacity of the value stream: items which should have been delivered are waiting in the bottleneck.


     



How Often do we Satisfy the Customer Requirements?: Process Capability

SPC will tell us if we have a stable process, then we can measure the probability that this process will satisfy, systematically, the customer requirements: this is process capability.

Indices Cp and Cpk are used to estimate this ability of the process to meet customer requirements.


Conclusions

  • Time is critical to everything we do
  • The right sequence of activities is essential
  • Value stream map: who does what and when in a sequential process

  • Update your schedule in real time to keep everyone in synch

  • Time variation causes queues and these cause waiting times increasing overall lead time
  • Reporting incidents the moment they happen enables a quick response
  • Statistical Process Control alerts operators of a significant change in the process in order to react before it is too late.

  • On-time delivery is usually a key customer requirement 














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