SPC, Moving Range Chart and Weighted Graph Basic Questions

[maghsoodi]

I have two questions in the field of statistics. Actually run across a problem with it. ?LENGHT OF MOVING RANGE? in IMR diagram in ?MINITAB? program, Have chosen to default 2. My question is, "How long can we change this number?",? Our criteria for what can be changed??. Please tell me if it is other than n = 2 for an example of the range of motion had to be defined.
My second question is on weighted graphs, including:?CONTROL CHART/TIME WEIGHTED CHART/MOVING AVERAGE
The difference ?LENGHT OF MA? and ?LENGHT OF MOVING RANGE? What is the criteria for determining the number ?LENGHT OF MA?, what is?
I mean, assuming you order - we define the value 4. The reason for choosing these numbers define the number 5 ,is what? What was the reason for these numbers?

 

[Bill McNeese]

A moving range of 2 is usually used in the I-mR (individuals control chart). A control chart is most effective usually when you minimize the opportunity for variation to occur n the range chart and maximize the opportunity for variation to occur in the X chart. This makes the X chart do the work. So, with individuals, there is less opportunity for variation to occur with n = 2, than with n = 4 or some other number.

When using an I-mR chart, you want a distribution that fairly symmetrical (normal-like), although this type of chart is pretty robust for most distributions. But if you have a heavily skewed distribution (for example, towards zero like in waiting times) and you don't want to fit the data to another distribution, you can use moving averages to help make the distribution more "normal." Subgroup averages tend to be normally distributed when n is large enough. Most the time, a value of n = 4 or n = 5 is sufficient.

There are other reasons why people will use other values of n. Some might use n = 3 to see how a variable changes over the last three months. Moving averages smooth out the variation - but I haven't used a moving average/range chart in a long, long time. Perhaps some folks here use them and can give you examples of where they used them and why the selected the value of n they did.