In DMAIC, the working methodology of Six Sigma, next comes Measurement Phase.
Measurement is quantifying anything into numbers. So when anything is expressed in number then we say that we have measured that thing. This helps to relate the objects to other objects in comparitive terms. For example if we note the time taken for a particular process to completethen we say that we have measured the completion time of the process. After that we can compare the process to other similar processes and relate as well in terms of time taken for completion.
According to Harington1 the measurement scales are of four types:
Nominal values are presence or absence of some attributes. Like success or failure.
The ordinal values are comparitives. Like one process is longer than other one.
Interval is the difference between two points of measurements. Examples are time, weighing scale etc.
Two quantities in relation to each other. Mathematical operations can be applied on these. For example length, time and weight.
Properties of Measurement
In order that the measured data is useful for the process improvement, it should be reliable and valid.
Reliability of Data
The data is reliable if we can reproduce the same data with the same methods again and again, even if taken from different sources. The lesser the bias from the same value, more the reliability.
Validity of Data
The data is said to be valid if our are of interest is covered sufficiently so that area of interest is represented fully.
Steps in Measurement
There are main two steps in data measurement related with a process:
- Data collection plan.
- Implementation of Data Collection Plan.
Data Collection Plan2,3
The data collection plan has various subparts. These are listed below:
- What to measure.
- Type of measurement.
- Type of Data.
- Operational Definitions.
- Data Collection Forms.
Implementation of Data Collection Plan
The data is collected as per previous step and then it’s used to calculate the baseline sigma. Baseline Sigma is the number of defects per million. In other terms deviation(variation) is measured from mean value.
1The content help has been taken from the book â€œThe Six Sigma Handbookâ€ by â€œTHOMAS PYZDEKâ€.
2The ideas in this book are presented with the help of the book: â€œSix Sigma For Everyoneâ€ written by: George Eckes.
3The Six Sigma Project Planner By Thomas Pyzdek.
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