Histogram Number of Bars

Jay Arthur
888-468-1537
303-756-9144
KnowWare International, Inc.
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Denver, CO 80224

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One of the most frequent questions we get is: "How do you select the number of bars on the histogram?"

The simple answer is we round the square root of the number of data points. For example:

• 25 data points = 5 bars
• 100 data points = 10 bars

If there are too many bars (e.g., more than 50) to display nicely on the page, we limit the number of bars.

Juran's Quality Control Handbook provides these guidelines for the number of bars and states that they are not "rigid" and should be adjusted when necessary.

Number of Data Points	Number of Bars
20-50	6
51-100	7
101-200	8
201-500	9
501-1000	10
1000+	11-20

The QI Macros prompts you for the number of bars you would like, however, you won't always get exactly the number you input. Why? Because we're trying to fit the graph to the page between varying specification limits within the constraints of Excel while still making it as readable as possible.

Compare the QI Macros histogram output to Minitab and you'll see that they are similar:

Download a free pdf of histogram formulas and sample calculations at histogram-manual-calcs.pdf

Other Histogram Information

Create a histogram in Excel using the QI Macros

Cp, Cpk and other process capability formulas

The histogram is just one of the tools included in the QI Macros for Excel SPC Software for Excel.

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