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.

### What is Z value in Six Sigma?

Z value is defined as the number of standard deviations a value is away from the mean.

### 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.

### How is Z lt represented in long-term data?

For long-term data, Z lt is represented as Zlt-total.

## Z long-term

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.