As we keep walking down this wonderful world of 6Sigma it’s important that we talk about how capability is measured. We’ve been talking about process capability for a few weeks now, so let’s talk about the capability measurement methods. This week we are going to focus on capability index and process capability.
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.
In our conversations about process capability, I want to focus your attention on baseline performance. Baseline Performance is an alternative way to view long-term and short-term data. When you hear baseline performance it most likely will be a description of baseline performance and it most likely will be used to describe long-term data.
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.
One of the key things learnt from 6Sigma is the ability to accurately measure and analyze the information your organization collects. This can be as technical or as general as your organization needs, the key is to understand the level of specificity your organization needs and analyze from there. A Black belt will be able to give you in depth analysis, but a good one will give you exactly what your organization needs. We’ll start the discussion with Multi- Vari Analysis.
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.
We all know my affinity for MSA but it wouldn’t be fair if we didn’t talk about the measurements for a bit. Six Sigma is built on measurements and the corner stone of effectiveness is to have measurements that are appropriate. So let’s dig in and figure out what defines appropriate measures.
What makes it appropriate?
There are four key areas to consider when you are trying to determine if your metrics are appropriate:
- Is it sufficient?-When you consider this you will need to look at how available the metric is. Ask yourself if you can readily gather the data. If you have to collect it and the collection times require more energy and resources than you can give, it may be time to rethink this metric.
- Is it relevant?-What will this metric tell you? Does it help you understand or identify your problems? If it doesn’t then maybe you need to take a step back and figure out what you need your metric tell you.
- Is it representative?-When you are looking at this metric, you should see a balanced representation of the people and the steps involved in your process. If you can’t see these things, take another look at your goals. Are you measuring the right things?
- Is it contextual?-When this information is put together with all of the other information you collect, do you see the big picture? In other words is the data painting a picture that makes sense to your and the people involved?
So MSA like everything else in Six Sigma is a tool and the thing that we need to remember is that for it to be effective, we have to make sure we are using it appropriately. Check your systems and let me know how they are working. If they aren’t working, give us a call.
In Six Sigma we are always collecting data, generally we are collecting data to address a current problem in our operations or services. The wonderful thing about Six Sigma is that we are also able to collect passive data. The usefulness of passive data is that it provides us with the ability to identify patterns, the catch to visualizing these patterns is in selecting the right graph to view the data.
Why use a graph?
The first benefit that comes to mind is the ability to see the error trends from a visual perspective. The other reasons graphs are a great tool are:
- Alongside identifying trends, they also help you see potential variable relationships. When you have a situation that could have multiple culprits, a graph can help you see which ones are a real potential.
- They can help you identify the risks that your customers will determine critical. This move allows your customer to be proactive instead of reactive, a much more desirable trait.
- It allows you to systematically dismiss variables and determine which one’s control other ones.
- It shows you the results of the passive data you’ve collected.
Where do I get the information for a graph?
Data is everywhere right? Yes and No. Your graph is only as good as your data, so we don’t want questionable data. The integrity of your data will be defined by your individual organization, but if you stick to these three questions you should be fine:
- What do you need the data to tell you?
- How often do you need to collect it?
- How do you need to collect it?
Next week we will get into the types of graphs and what times of data are appropriate for them. Until then happy hunting!