Definition of Z lt:« Back to Glossary Index
When working with process capability, it is important to know if your process can hold up to any deviation. Z lt can help us determine that.
Overview: What is Z lt?
Z lt stands for Z long-term. It is a representation of the overall process capability and can be utilized in the determination of a current process probability in making out-of-spec parts. It is one of several valuable indices in Six Sigma.
3 benefits of Z lt
There are some clear benefits to working with Z lt:
1. Valuable in determining Ppk
You can determine Ppk by dividing Z long-term by 3.
2. Needed to find the Z Shift
You can find the value of the Z Shift by finding the difference between Z short-term and long-term.
3. A better measure of reality than Z short-term.
Z long-term can be a better measure of reality than Z short-term as it focuses on a period of time.
Why is Z lt important to understand?
Here are some reasons to understand how Z lt works:
1. Allows for accurate estimates easily
Being able to understand Z long-term provides engineers with a tool for estimating long-term process variability without having to use laborious tables or a computer.
2. Out-of-spec parts
Understanding Z long-term can give you knowledge as to whether your current process will stand up to creating out-of-spec parts.
3. Project management
A knowledge of Z long-term and its ability to help analyze process capability can be a useful tool in project management.
An industry example of Z lt
A sampling is done in a manufacturing plant of parts they produce that are used in making commercial dryers. The sampling shows that a number of the parts are out-of-spec. Z long-term is utilized to determine if the process is able to maintain its integrity with the amount of out-of-spec parts that are being created.
3 best practices when thinking about Z lt
Here are some things to think about when working with Z long-term:
1. The larger the Z shift, the better
When you find the Z shift as the difference between Z long-term and short-term indices, the larger the Z shift you end up with means the more room there is to improve the process. You have more leverage in eliminating shifts and drifts within the process mean.
2. Different calculations
There are different calculations for Z short-term and Z long-term. Knowing the differences is important in making proper calculations.
3. Meeting specs in the long-term
Your customers care more about your ability to make products up to spec over the long term. Putting more of a focus on Z lt than Z short-term might be in your best interest as short-term may be harder to control.
Frequently Asked Questions (FAQ) about Z lt
1. What is Z value in Six Sigma?
Z value is defined as the number of standard deviations a value is away from the mean.
2. What is Z in process capability?
In process capability, Z is the point where the area to the right of that point is equal to the proportion of defective units in a process.
3. How is Z lt represented in long-term data?
For long-term data, Z lt is represented as Zlt-total.
Z long-term is an important aspect in knowing how much room there is in improving processes and knowing the full scope of your process capability over a period of time.« Back to Dictionary Index