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Skewed distribution

Skewed data is quite simply, a data distribution that is not symmetrical. Usually the longest tail points should point in the direction of the skew. Here’s what a skew looks like

Skewness can be caused by any variations, but here are the most common causes.

Natural Limits

Natural limits-these are the limits of sample size. The problem with natural limits is that these natural limits can bias the estimation of results and in some cases ensure that there can be no specific correlation between the sample and the data field.

Sorting

This is also known as artificial limits and it’s important to realize that limits are imposed by the person analyzing the data. Basically artificial limits set an arbitrary point for acceptable and not acceptable. Say you make 40 chairs and hour, your designer decides that any chair that doesn’t make a rating of 80 is unacceptable. That acceptable rating is completely arbitrary based on the designer’s standards.

Mixtures

Mixtures occur when data from different sources is expected to be the same and is different. Say you’re looking for error data from two cashiers Shift A credit card receipts and Shift B, cash receipts and the skew is not the same. You were expecting the error rate for each method to have a normal distribution and what you got showed something like this.

Next week we will pick up with a continuation of non-normal distributions. Until then, Happy analyzing

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First, what is a distribution?

Simply put, a distribution will tell you how often a variable occurs in your process. This is important because the commonness of your variables will inevitable create a foundation for your improvement project.

Types of Distribution

The Normal Distribution

A normal distribution (Gaussian Curve, the average person knows it as the Bell Curve) shows a equal distribution. The mean (the average) divides the data in half, 50% on the data on each side of the mean. The Normal Distribution will have the following hallmarks:

This distribution is considered to be the most important distribution.

The area under the curve should equal 1.

Physical aspects of the curve should resemble a hill and should be symmetrical.

Both directions on either side of the mean extend indefinitely and never touch the horizontal axis.

White noise in your process should produce a normal curve shape

The Z distribution has a mean of 0 and a standard deviation of 1.

The mean (average), median (mid-point) and the mode (most common value) should be the same data value.

Next week, it’s on to non-normal classifications.  Get to analyzing and if you need any help, reach out and let us know!

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Variation Sources

Within Unit Encoding

This variation source occurs when you are measuring output from a single production cycle. Some places that variation is likely to occur are the width of parts, color shading, length of toy etc. Now you can choose to analyze different production cycles on the same day or alternating days, but you will always be comparing samples from the same cycle. A new production sample means a new data point.

Between Unit Encoding

These names are dead giveaways, but I digress! This implies that you are looking at samples from two different production cycles. This is different in that you would want to identify two different samples from different production cycles. The variations you are looking for will give you some clue as to whether the variations are operation influenced or process influenced.

Temporal Encoding

This is the trickiest variation source. This specifically calls for you to compare your variation averages from all of your data points in a single day. So you can theoretically have both within unit variation data and between unit variations data, depending on how specific you need to get.

The key to getting the most out of your data is to understand what it’s telling you. Understanding where the variations are coming from is the first step to getting the most out of your data.

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What does it mean?

The first thing we need to understand are the terms for measurement, so here are a few basic definitions.

Cpk and Cp are capability rates and Pp and PPk are performance rates.

Cp- When you see this, you’re talking about rate of your process capability. To find it you use this formula:

Pp-When this comes up, the conversation is speaking to the pure performance of your process. The formula to find this data is:

Cpk- This refers to your process capability index, basically telling you how close your project is running to the acceptable limits. The formula for finding Cpk is:

Ppk-This refers to the non-centered distribution, when you hear this term it’s referring to adjustments to the effects that distribution. The formula for Ppk is:

What’s the Difference?

The main difference is the way the information is calculated. Cp and Pp is really short term data that considers only the quantity of information determined by specified limits. Cpk and Ppk rates process capability based on centralization and variation within one specification limit.

Data is so much more than numbers, but by understanding the why and the how 6Sigma begins to teach us what is significant in our data.

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What it means

Baseline in a nutshell gives you the average long-term performance of a specific process without

controlling any variables. The easiest way to think of this is a visualization of FTY (First Time Yield). Remember FTY shows you the challenges in your process when they are normally run without any interference from you.

What to use it on

When measuring baseline, you are identifying a typical challenge within a process. For example if you are observing the process for returns, your long-term data will include morning, afternoon and evening shift; multiple employees and submission points (email, in-person and via telephone).

Your short term data will appear on the visualization as well, so you will be able to see in a visual representation short-term and long-term average behavior for your processes. If there is always a dip in quality at around lunchtime, you will be able to see that visually represented in your data.

Why use it?

Baseline performance is going to quickly tell you where your burning platform issues are. If you are heading into a meeting with management, this is a report to take with  you. It shows the long-term vs. short-term and gives you solid business evidence to support improvement projects.

Next week, we will tackle measures of capability and what they tell you. Remember that this is can be the starting point to discuss improvement with your belt. If you need to get started, give us a call and we can get you started.

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• What part of your business is making the requirements? Are you responding to customer service issues? Are you responding to due diligence requirements or compliance issues? Are you redesigning a product?
• How stable are the requirements? Is this a validated process or is it likely to change in the near future?
• How does your staff understand the process? Is information relayed directly to the personnel using the process or is it a trickle down environment?

Before you even begin to consider how to change the way you collect your data, you have to understand how it’s currently being done. The first thing to think about during capability studies is that when a capability study is conducted all of the information is included in the sample data; because of this you need to have a good understanding of short-term data and long-term data.

Short term data

• Is data that is collected during a very short, very specific period of time. For instance you may be looking for the errors that occur during the late shift on Wednesday.
• Is generally free of special cause variation.
• Commonly represents best case performance.
• Generally has more than 30 data points.

Long-term data

• Collected for a longer period of time, usually monthly or quarterly, through various periods of time.
• Contains common and special cause variations.
• More accurate representation of performance.
• Generally has more than 100 data points.

Understanding the way you collect data helps you make the most accurate analysis and leads to more refined business decisions. Understanding data can give you the tools to empower your employees in a meaningful way, taking the emotion out of business and offering a chance for data driven decisions.

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What is Multi-Vari Analysis?

Simply put this puts a face to the data. Once you have collected all of your information Multi-Vari studies take the data and illustrate the patterns of variation within the data. It helps you identify group or correlations between subgroups and over time.  When you can identify the groups, you can make assumption or draw conclusions based on the data. For example if your data shows the your staff made more errors on product X you can draw the conclusion that your improvement efforts need to be focused on that particular product.

What is it used to assess?

Multi-Vari studies are useful in many ways but the most standard uses are

• to illustrate data in graphics.
• to show how work is influence by defined variables.
• to show the impact of specific material, departments or methods.
• the effects of external factors such as noise, delivery delays etc.

When you need to show stakeholders, influencers or project staff what you have found multi- vari studies are a great way to produce a visual. Since most people learn by doing, a visual representation allows them to see what they have done and to show leadership the gains or losses accordingly.

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What is a Capability Study?

To review from last week, a capability study is a way to ensure that your process is consistent over an extended period of time. For example if step 3 in your process produces 3 errors per cycle for 3 years, your process in consistent.

How Do You Find Stability?

There are a ton of tools you can use to test the stability of your process, but some of the most common tools are Time Series Plots and Control Charts. In addition to these tools there is a step by step process (of course!) to test the capability of your process, here they are.

What should know about capability studies?

As with all 6Sigma tools, the effectiveness of this tools lies more in how you understand and how you apply it. The most important things to remember are:

• Capability studies are used to measure the same parts of the process, at the same stage in the process at exactly the same time every time it is measured.
• You can use the capability study on discrete and continuous data.
• You get the best (ie most meaningful) information when you run the study on already stable and predictable data. New processes are not the best place for this tool.
• When you hear Sigma Level, they are talking about capability.
• Capability studies require you to understand:
  • The limits of your customer or organization.
  • The difference between short-term and long-term
    data and what those differences mean to your organization or customer.
  • Mean and standard deviation.
  • How to assess normality of your data.
  • How your organization or customer determine Sigma level.

Capability Studies can give you a great deal of insight on how your organization is running and what is making it difficult.  This is one way to get a sense of the information flow and the quality of the information you can get your hands on. So let’s start off the new year with a look at what your data is telling you. Happy Hunting!

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What is Process Capability?

In a nutshell Process Capability is:

• What it takes for your process to meet your customers’ needs right out of the gate with no modifications. This means for lack of a better term, inherent perfection.

• The information that can be provided on centering, variation and inappropriate measurement limits.

• The baseline metric for improvement

When determining your process capability there are three types of capabilities that we analyze:

• Continuous Capability- If you process is capable and in control, ideally you should get your desired outcome. This analysis measures the life cycle of your process telling you if the process has continued to be capable and in control.

• Concept of Stability-The idea of stability is the ability to answer the question ‘will my process produce the same result at this step every time it is used?’ To be technical, stability measures the ability of your process to meet its requirements at a regular and specific interval.

• Attribute Capability-This analysis makes assumptions about your data and is always long term data.

This week we’ve just scratched the surface on Process Capability. Next week, we’ll start digging a little deeper and show some illustrations of what it looks like.

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