iSixSigma

F-Chart

Definition of F-Chart:

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An F-Chart is a simple chart to generate and interpret, especially if you are familiar with comparable charts such as the P-Chart.

Overview: What is an F-Chart?

An F-Chart is a control chart that is utilized to monitor the number of defective units in each sample, provided the sample size remains the same. The F in F-chart stands for “faulty”. It is also known as an NP-chart. With an F-Chart, items are broken down into two basic categories, an equivalent of “pass” or “fail”.

3 benefits of an F-chart

Here are a few key benefits of an F-Chart that are worth noting:

1. Simplicity

This chart is especially easy and simple to create since the number of defects are plotted directly on the chart.

2. It is faster to make than comparable charts

Being that you are not calculating the proportion of faulty parts, this control chart is quicker to produce than comparable charts like the P-chart.

3. Long-term monitoring capability

This chart can be used over time in order to monitor the stability of a process, which allows for identifying and correcting the instabilities.

Why are F-Charts important to understand?

There are a number of reasons why F-Charts are important to have a solid understanding of:

1. They are a key tool

An F-Chart can be used to monitor if a process is stable enough to meet standards or not. This makes it a key statistical quality tool in industries like manufacturing.

2. The data is simple

An F-chart is easy to understand as it records only simple, binary data such as “conform” or “non-conform”, “faulty” or “non-faulty”. Its ease of interpretation makes it worth the short time required to comprehend it.

3. The control lines allow for spotting issues immediately

If you know how to interpret the chart, you can immediately spot when a process has gone out of control with too many defects. This is beneficial as you can catch where processes may need improvement as well as if there may be any special causes that need investigating.

An industry example of an F-Chart

A supervisor in the machine shop of a large manufacturing plant is tasked with monitoring the number of machines that are out of service over a period of two months. The supervisor creates an F-Chart, considering any out-of-service machine as a defective unit.

In the chart, it is shown that the average number of machines that are out of service on any given day is five. On day 15, it shows that an out of control number of machines were not functioning correctly. The supervisor investigates with the machine shop staff as to whether there were any special causes that day.

3 best practices when thinking about an F-Chart

If you are going to create an F-Chart, here are a few key practices to help you along:

1. Items need to be categorized into just two groups.

With this kind of chart, items need to be put into just two groups, such as “pass” and “fail.” If you only need to determine if an item is defective or not, this chart works well. For instances where you need to count the number of defects per item, another chart would work better.

2. Have your data in the correct time order.

The order of your data is important since changes are detected in control charts over time. Have the oldest data on top.

3. Collect data at regular time intervals.

Data collection should occur at equally spaced intervals of time. Make sure the intervals are at a short enough time to note process changes as they happen.

Frequently Asked Questions (FAQ) about F-Charts

1. What is the difference between this chart and a P-chart?

A P-chart plots the proportion of items while an F-chart plots the number of items.

2. Can sample size vary in an F-Chart?

No. The sample size must be the same for an F-Chart.

3. If I cannot use an F-Chart for varying sample sizes, which chart can I use?

You can use the P-Chart since it focuses on the proportion of defective items in a sampling. Control limits do not need to be recalculated each time the sample size changes as long as it falls within +/- 20%.

Adding the F-Chart to your toolbelt

With F-Charts being so simple to generate and easy to interpret, you should absolutely know how to work with them. They can be extremely valuable for monitoring defects and seeing where processes may need improvement.

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