5 Simple Steps to Conduct a Non-Normal Capability Analysis
In transactional or service processes, the data distribution most often found is non-normal.
In most cases, we are looking at lead-time data.
For example, our Lean Six Sigma project is to reduce the lead-time to install an I.T. application at a customer site. It should take no more than 30 days (working 10 hours per day Monday – Friday) to complete, test and certify the installation. If we follow our standard Consists of input, value-add, and output. Learn More..., the target lead-time should be around 24 days.
While 24 days is our target, the customer has expressed that their satisfaction increases as we perform the installation faster.
We need to understand our baseline capability to meet customer demand. Therefore, we can perform a capability analysis.
We know our data should fit a non-normal (positively skewed) distribution. It should resemble a ski-slope like the picture below:
I will teach you five simple steps to understanding the capability of a non-normal process to meet customer demands.
Step #1 – Collect data
Collect the data from the process.
In this scenario, we are collecting sample data. We pull 100 samples that covers the full In statistics, the range of a set of data is the differenc... Learn More... of variation that occurs in the process.
In this case the full range of variation comes from three installation teams. We will take at least 30 data points from each team.
Click Here to download the data in an MS Excel Spreadsheet.
Step #2 – Identify the Shape of the Distribution
We know that the data should fit a non-normal distribution. As Lean Six Sigma practitioners, we must prove our assumption with data.
In this step, we conduct a normality test to prove non-normality.
Copy the LeadTime data from the Excel Spreadsheet into a Minitab worksheet.
Run the “Normality Test” found under “Stat > Basic Statistics > Normality Test … ”
Populate the “Variable:” with LeadTime and Click OK (See example below)