Intervals & Tests
I have a column of 300 numbers that I would like to do a histogram, Johnson bounded curve fit, but the chart that is generated is a normal curve and the program states the selected region could not be fit into a Johnson curve fit What do I have to do to make it fit? I reduced the number of rows to 100 and it worked, but I need all the data if possible.
As I expected, the data is out of control. When you are dealing with n=1 (individuals) data, you need to use some other chart than the Individual-X chart to establish the control of the process, since you need to assume a distribution for the Individual-X chart and you cannot fit a curve until you have established the process is in control (see also: http://www.qualityamerica.com/knowledgecente/knowctrDefining_Control_Limits.asp and associated links).
I used a Moving Average chart with subgroup size 1 and moving size 3. The process was out of control in this scenario, but I noticed groups 205-300 were in control. That is not a lot of data to fit a curve (you would like 300 observations), but it is enough to make it interesting. When I plotted those groups by themselves on the Moving Average chart, they were in control:
Based on this chart, I then plotted all the groups 1-300 on the Ind-X chart and let the software fit the control limits based on groups 205-300 (in the Control Limits tab), specifying a Johnson distribution in the Analysis tab:
This confirms the process out of control, as evidenced on the Moving Average chart.
The key point is that you cannot expect to fit a single distribution to an out of control process: if the process is out of control, then (by definition) there is more than one statistical distribution present in the data.
Learn more about the Statistical Inference tools for understanding statistics in Six Sigma Demystified (2011, McGraw-Hill) by Paul Keller, in his online Intro. to Statistics short course (only $89) or his online Black Belt certification training course ($875).