Lean Six Sigma For Dummies. Martin Brenig-Jones
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Название: Lean Six Sigma For Dummies

Автор: Martin Brenig-Jones

Издательство: John Wiley & Sons Limited

Жанр: Экономика

Серия:

isbn: 9781119796732

isbn:

СКАЧАТЬ unit as the “thing” you are measuring; in this case we’re talking about days.

      © Martin Brenig-Jones and Jo Dowdall

      FIGURE 1-2: Histogram showing the time taken to process orders.

       x in the formula represents your individual data points

       xi represents each x from x1 up to xn

       n represents the number of data points in your data set

        represents the average (mean) of your data points

        represents the “sum of”

      © Martin Brenig-Jones and Jo Dowdall

      FIGURE 1-3: Standard deviation.

      As with most things in life, tackling the formula in stages makes it easier, so let’s have a go.

) of those data points, and the mean (
) is 7 (because
). Are you with us so far? Great!

      Next we need to work out the

part. This means subtracting the
value (7) from each of our data points.
, and so on. All is going well, but we now have some negative values in the mix, and we need to get rid of them. This is done by squaring the numbers we’ve just worked out. So
, and so on. Next, add together all of the answers we just got. If you’re still with us and have done that bit, you should get 10. So now we have real numbers to work with, and the numbers that we put into the formula are
(Yes, we did all that work to get to 10 divided by 5.) 10 divided by 5 is 2. We then, finally, take the square root of 2 to get the standard deviation value of 1.41. Hooray!

      Note that there are two versions of the standard deviation formula included in Figure 1-4. The first is used when we have a sample of data, and the second is used when we have the entire population. The data set we used to work through the example above was very small! If we were using the formula for a sample, instead of using n we’d use

. So we’d end up with
, which is 2.5. The square root of 2.5 is 1.58.

      © Martin Brenig-Jones and Jo Dowdall

      FIGURE 1-4: Standard deviation formula.

      In practice, when the sample size is more than 30, there’s little difference between using n or

. When we refer to a “population,” this could relate to people or things that have already been processed, like, for example, a population of completed and dispatched policy documents.

      

Now for some good news: You can use a scientific calculator, Excel, or any number of online calculators to work out the standard deviation of your data, without having to worry about this formula.

      Considering customer requirements

      So far so good, but without understanding the customer requirements, it’s not possible to tell whether cycle time performance is good or bad.

      Let’s say the customer expects delivery in five days or less. In Lean Six Sigma speak, key customer requirements are called CTQs, (Critical To Quality). We discuss CTQs in Chapter 2 and describe them in more detail in Chapter 4, but essentially they express the customers’ requirements in a way that is measurable. CTQs are a vital element in Lean Six Sigma and provide the basis of your process measurement set. In our example, the CTQ is five days or less, but the average performance in Figure 1-2 is four days. Remember that this is the average; your customers experience the whole range of your performance.

      

Too many organizations use averages as a convenient way of making their performance sound better than it really is.