STANDARD DEVIATION (A SIMPLE DEFINITION):
Many of the people have doubt about the definition of standard deviation for a process.. I asked many to explain this. But after reading many books, I have understood something, which I want to share with..
Here goes..
Deviation = move away from the target. Putting like this “Standard deviation means moving away from the target which is constant“.
First of all deviations are inevitable in a process & cannot be make null.
Why the deviation happens generally in a process?
Deviation in the process mean: the finished product cannot have the dimensions which we expected .i.e. for suppose if I designed a shaft of diameter 7 mm. After producing it in the production line it will be either more or less than 7 mm.
So, First we should know why the deviation happens in a process in order to understand the magnitude of standard deviation.
Deviation in a process can be due to two type of causes:
But chronic causes are inbuilt in nature. From the day one the process starts, they remain in the process & cannot be eliminated .This can be due to (6M) man, machine, method, material, measurement, mother nature..
So, lets see in detail:
Deviation due to man:
A person who is operating a machine, try to keep the settings perfectly which will end up with adjustment of the setting all the time. This will create the variation in the process ( Or the deviation of the product from the desired dimensions)
Deviation due to machine: Any machine, how sophisticated & brand new it is, it will have some inbuilt limitations (examples like: wearing of parts, rubbing of parts, vibrations, heat generated) will create deviation in the process.
Deviation due to method:
Ever person cannot be expected to do in the same way...Even though SOP (Standard operating procedure) is being followed, the same method of producing the product cannot be achieved... This type of deviation is due to knack & intuition of a person who is working. An experienced person will know when he need to add the ingredients at what time & at what rate which locals cannot..
Deviation due to material:
This I would like to explain through an example. The same example which I explained in the starting..If I decided to produce a shaft of 7 mm diameter with high degree of finishing then I do turning followed by finishing operations..Even after rigorous grinding techniques, if the surface of shaft Is kept under microscope, it can be seen that still there will be some protrusions..This is inbuilt of the material which will create the deviation..Even a precise CNC lathe machine cannot produce accurately..
Deviation due to measurement:
Here comes with the repeat ability & reproducibility of the measuring instrument.. Any measuring instrument cannot measure accurately & even though if it gives, it again depends on the person who is using it , which will create the deviation in the process.
Deviation due to Mother Nature:
Mother Nature includes the surroundings where we perform the process. Some of the effecting features are humidity, temperature, ambiance, light, sound, amount of air & some special requirements for some particular products..Like humidity percentage greatly affect the quality of product.
So, Standard deviation tell us that how the 6M (man, machine, method, material, measurement, mother nature) is influencing the process & if there are any special causes influencing..Even though it sounds simple, it is difficult to reduce these deviations..
If standard deviation is increasing for a process, it shows a negative impact on the process. To make it to minimum, we should know the causes in which we need to bifurcate into chance & special. By focusing on the causes we can reduce it to minimum & cannot be zero.
Many of the people have doubt about the definition of standard deviation for a process.. I asked many to explain this. But after reading many books, I have understood something, which I want to share with..
Here goes..
Deviation = move away from the target. Putting like this “Standard deviation means moving away from the target which is constant“.
First of all deviations are inevitable in a process & cannot be make null.
Why the deviation happens generally in a process?
Deviation in the process mean: the finished product cannot have the dimensions which we expected .i.e. for suppose if I designed a shaft of diameter 7 mm. After producing it in the production line it will be either more or less than 7 mm.
So, First we should know why the deviation happens in a process in order to understand the magnitude of standard deviation.
Deviation in a process can be due to two type of causes:
- Sporadic or special or Assignable causes
- Chance or chronic or common causes
But chronic causes are inbuilt in nature. From the day one the process starts, they remain in the process & cannot be eliminated .This can be due to (6M) man, machine, method, material, measurement, mother nature..
So, lets see in detail:
Deviation due to man:
A person who is operating a machine, try to keep the settings perfectly which will end up with adjustment of the setting all the time. This will create the variation in the process ( Or the deviation of the product from the desired dimensions)
Deviation due to machine: Any machine, how sophisticated & brand new it is, it will have some inbuilt limitations (examples like: wearing of parts, rubbing of parts, vibrations, heat generated) will create deviation in the process.
Deviation due to method:
Ever person cannot be expected to do in the same way...Even though SOP (Standard operating procedure) is being followed, the same method of producing the product cannot be achieved... This type of deviation is due to knack & intuition of a person who is working. An experienced person will know when he need to add the ingredients at what time & at what rate which locals cannot..
Deviation due to material:
This I would like to explain through an example. The same example which I explained in the starting..If I decided to produce a shaft of 7 mm diameter with high degree of finishing then I do turning followed by finishing operations..Even after rigorous grinding techniques, if the surface of shaft Is kept under microscope, it can be seen that still there will be some protrusions..This is inbuilt of the material which will create the deviation..Even a precise CNC lathe machine cannot produce accurately..
Deviation due to measurement:
Here comes with the repeat ability & reproducibility of the measuring instrument.. Any measuring instrument cannot measure accurately & even though if it gives, it again depends on the person who is using it , which will create the deviation in the process.
Deviation due to Mother Nature:
Mother Nature includes the surroundings where we perform the process. Some of the effecting features are humidity, temperature, ambiance, light, sound, amount of air & some special requirements for some particular products..Like humidity percentage greatly affect the quality of product.
So, Standard deviation tell us that how the 6M (man, machine, method, material, measurement, mother nature) is influencing the process & if there are any special causes influencing..Even though it sounds simple, it is difficult to reduce these deviations..
If standard deviation is increasing for a process, it shows a negative impact on the process. To make it to minimum, we should know the causes in which we need to bifurcate into chance & special. By focusing on the causes we can reduce it to minimum & cannot be zero.