Within Lean Six Sigma projects we have a range of data analysis tools we can use to analyse and interpret data to understand what is happening in the services, products or processes we have observed. These data analysis tools include Pareto analysis, Histograms and Scatter Charts among others. Within this article, we are going to explore the basics of Histograms.
What is a Histogram?
A Histogram is a chart or type of graphical analysis tool that can be used to understand data and information. The Histogram shows the frequency of numerical data using rectangles where the height of the rectangle demonstrates the distribution frequency of a variable being measured. The width of the rectangle demonstrates the value or variable.
For example, on the Histogram below of people’s heights. We can see out of 15 people who had their height measured 9 of them had a height between 1.73 meters and 1.91 meters. The vertical (Y) axis of a histogram always represents the frequency, whereas the horizontal (x) axis will be the variables you are measuring, such as height, minutes, or instances.
Note: Not all rectangles of a histogram have to be the same width as you would see on a Bar chart.
What’s the difference between a Histogram and a Bar chart?
The Histogram displays distribution frequency as a two-dimensional figure, which means the height and the width of columns or rectangles have meanings and both can vary. Whereas a bar chart is a one-dimensional figure where the heights of the bars represent something specific, and the width of the bars has no meaning. On a Histogram there are no gaps between columns unless there is no frequency data in that range, column widths change as the variables represented change. Whereas, on Bar charts, the bars are all the same width with gaps in between them to separate the data.
When to use a Histogram
Histograms are used as useful data analysis tools for all types of data, first, you need to understand what data type you have. Histograms are to be used when you have continuous measurements to understand the distributions of values and to look for outliers. Another option for this data type might also be Box Plots. Histograms take continuous data and place them into ranges of values known as bins. Each bin is represented by a bar or rectangle that has a count or percentage of observations that fall within that bin.
You may want to use a Histogram for comparing two different data sets to evaluate the distribution and the central tendency of the data to help understand a problem. This can often be useful when measuring the outputs of a process and comparing them with a customer’s spec limits to see the distributions of outputs a performance against the customer’s spec. Experiments can be conducted on the inputs and the process and measuring the output to see if the improvement positively impacts the output.
Download the free Histogram excel template to use on the data analysis of your lean six sigma projects. All you need to do is input your data and the formulas will automatically display the data in the Histogram for you to analyse and interpret.
In conclusion, if you have continuous data that you need to analyse and interpret Histograms are an ideal tool to use to understand the distribution and centring of the data to start to understand what type of problem you might have and how you can address it. There is so much more to cover within Histograms, therefore this page only covers the basics for now to give an understanding of how to graphically analyse continuous data within Lean Six Sigma projects.