Are My Data Normally Distributed?

You have a set of data. You would like to know if the data comes from a normal distribution. How do you do that? A normal probability plot can be used to determine if sets of data come from a normal distribution. This involves using the probability properties of the normal distribution. The data are compared to a normal distribution in such a way that will result in a straight line if the data are normally distributed.

For example, consider the normal probability plot below. This is a data set of the forearm lengths for males. We want to know if the data come from a normal...

x control chart for consistency

How Can We Trust Our Measurement Process?

Measurements tell us many things. They tell us if a product is within specification, if we met our goal, if we are staying the same, if we are improving, if we are getting worse, etc. What makes a good measurement process? This is essentially asking the following question:

What does it take for us to trust our measurement process?

Think about trust. Why do we trust some people? One reason is that they are consistent – we know what to expect from them because they are consistent. The same is true of our measurement processes. We begin...

How to Analyze a Cause and Effect Diagram

Our previous blog, What is a Cause and Effect Diagram , introduced the technique and presented the cause and effect diagram below on why a car will not start.

This blog addresses how you analyze a cause and effect diagram. The first step is to examine each idea and determine the degree to which the idea is an actual cause of the problem. For example, how likely is no gas to be the reason the car won’t start. Is it very likely? Is it somewhat likely? Or is it...

What is a Cause and Effect (Fishbone) Diagram?

A cause and effect diagram is a tool that shows the relationship between a quality characteristic (effect) and possible sources of variation (causes). As shown below, the effect could be a problem that needs to be solved or the goal of the process. The effect would then be listed on the cause and effect diagram. The causes involve everything that might trigger the problem. Cause and effect diagrams are also called fishbone diagrams (because of their shape) and Ishikawa diagrams (because of their developer).

The cause and effect diagram is one of...

How Do I Analyze a Scatter Diagram?

Our previous blog ( What is a Scatter Diagram? ) included an example of overtime in a warehouse. You are a warehouse manager, and your boss is concerned about overtime. You think that overtime is caused by the work level – the more lines picked in the warehouse, the more overtime. You constructed a scatter diagram to see if that is true. That scatter diagram is shown below.

How do you analyze this scatter diagram? There are several things you can do. First, you can simply look at the scatter diagram...

What is a Scatter Diagram?

A scatter diagram shows the relationship between two variables. For example, you might want to compare the speed you drive with the time it takes you to get to work, or to compare the heights and weights of children, or to compare the steam usage in a plant to the outside temperature. This is what scatter diagrams do.

Suppose you are a warehouse manager. Overtime is a concern to you since it is something your boss watches closely. There is overtime every day. You have a theory that the overtime is simply caused by the work level – the number of lines that are picked each day in the...

What Do These Histograms Tell You? - The Answers [VIDEO]

Our recent blog, entitled What Do These Histograms Tell You?, showed you five different histograms. The blog challenged you to figure out what the histograms were telling you about a supplier’s process. This is the background information we gave in our blog:

“A supplier sends in weekly shipments of one raw material to you. There is one raw material characteristic that is key to how it performs in your process. The supplier provides you with a Certificate of Analysis with each shipment that includes the measured value for this raw material...

What Do These Histograms Tell You?

Our previous blog introduced histograms. Histograms tell you four things about your process. Remember what those were? If not, see our previous blog, What is a Histogram? . Histograms also can give you insights to things you might not normally see. The example below demonstrates this.

Here is the background information for you. A supplier sends in weekly shipments of one raw material to you. There is one raw material characteristic that is key to how it performs in your process. The supplier provides you with a Certificate of Analysis with...


What is a Histogram?

A histogram is a snapshot in time of your process. It tells you four things:

  • Which result (or range of results) occurs most frequently
  • How much variation there is
  • What the shape of the variation looks like
  • If any results are out of specifications

Remember, all processes have variation. Processes in statistical control tend to form a stable pattern, which is called a distribution. Distributions are characterized by three parameters: location, spread, and shape. These parameters can be estimated from a histogram. An example of a...

What is a Pareto Chart?

A Pareto chart helps you answer questions. What is the reason for the most rework or scrap in your organization? Which customers complain the most? What keeps us from getting the books closed after the end of the month. These questions are common in organizations, but we don’t always agree on the answer. This is where the Pareto chart comes in. A Pareto chart is a data-based approach to determining what “vital few” issues you should be working on.

The Pareto chart is named after Vilfredo Pareto, an Italian economist, who discovered that 80% of Italy’s wealth was held by 20% of the...


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