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Σ or Sigma is the Greek upper case capital letter S and is used in mathematics to represent summation or addition of a series of elements or set of data values.
x1, x2, x3, ... xn is a set of numbers. x1 is the first number in the set, xi is the 'i'th number, xn is the last of n numbers.
Elements can be simply adding up a set, or more involved such as summing squares or other algebraic equations.
When most people hear the word average they think normal, typical, boring. But, in many cases that is very far from the truth.
Averages can not only be interesting, but also impossible!
Consider the call center that measures average response time. The average response time is less than their target, so they must be doing pretty good, right???
One of the things that I have noticed as I look out at the world is – things change.
The weather changes, the tides change, and customer demand changes.
Of these three, variation in customer demand is perhaps of the most interest to lean practitioners, because, after all, if customer demand were constant, the challenge of designing and managing the value stream would be much simpler.
So accepting that in most cases customer demand does change, a sensible follow up questions are: By how much? And is the demand at least stable?