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Histogram PDF Print E-mail

What does it do?

 Active ImageA histogram is a basic graphing tool that displays the relative frequency or occurrence of data values-or which data values occur most and least frequently. A histogram illustrates the shape, centering, and spread of data distribution and indicates whether there are any outliers. The frequency of occurrence is displayed on the y-axis, where the height of each bar indicates the number of occurrences for that interval (or class) of data, such as 1 to 3 days, 4 to 6 days, and so on. Classes of data are displayed on the x-axis. The grouping of data into classes is the distinguishing feature of a histogram

Why Use?

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It is important to identify and control all sources of variation. Histograms allow you to visualize large quantities of data that would otherwise be difficult to interpret. They give you a way to quickly assess the distribution of your data and the variation that exists in your process. The shape of a histogram offers clues that can lead you to possible Xs. For example, when a histogram has two distinct peaks, or is bimodal, you would look for a cause for the difference in peaks

When Use?

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Histograms can be used throughout an improvement project. In the Measure phase, you can use histograms to begin to understand the statistical nature of the problem. In the Analyze phase, histograms can help you identify potential Xs that should be investigated further. They can also help eliminate potential Xs. In the Improve phase, you can use histograms to characterize and confirm your solution. In the Control phase, histograms give you a visual reference to help track and maintain your improvements

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The pictorial nature of the Histogram enables to see the patterns that are difficult to see in simple table numbers are shown for information. The Bell shape in the graph shows whether the process is homogenous or not. The graph is based on statistics and shows central tendency and spread of the data.

 

 
 
Let us take an example now. Draw a histogram for the following information.
 
 
Height of student in feet Frequency(No of Students)
Relative Frequency
0-20
0
2-4
1 1
4-5** 4 8
5-6**8
16
6-8 2
2
   
 
  
 

(Ignore relative frequency for now). It is difficult to draw a bar chart for this information, because * * the class divisions for the height should be same, if not same calculate 'relative frequency’. You must work out the relative frequency before you can draw a histogram. To do this , first you must choose a standard width of the groups . Some of the heights are grouped into 2s (0-2, 2-4, 6-8) and some into 1s (4-5, 5-6). Most are 2s , so we shall call the standard width 2. To make the areas match, we must double the values for frequency which have a class division of 1 (since 1 is half of 2). Therefore the figures in the 4-5 and the 5-6 columns must be doubled. If any of the class divisions were 4 (for example if there was a 8-12 group), these figures would be halved. This is because the area of this 'bar' will be twice the standard width of 2 unless we half the frequency. The height is grouped 0-2, 2-4 etc, but not all of the groups are the same size. For example the 4-5 group is smaller than the 0-2 group.

 

When drawing a histogram, the y-axis is labelled for frequency’.

 

 

 
 
 
 
 



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