How to Determine Histogram Bin Intervals

To show a useful spread of data, you may need to estimate the ideal histogram bin interval. Here's how:

  1. Count the number of data points.
  2. Take the square root of the number of data points and round up to determine the number of bins required.
  3. Divide the specification tolerance (USL-LSL or Max-Min value) by number of bins.

For example: 25 data points = 5 bars or 100 data points = 10 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   Number of Data Points Number of Bars
20-50 6   201-500 9
51-100 7   501-1000 10
101-200 8   1000+ 11-20

Three Reasons to Use the QI Macros to Create a Histogram vs Manual Calculations or Using Excel by Itself

  1. Ease of Use: Its easy to create a histogram in the QI Macros. Just select your data, click on the QI Macros menu and select Histogram. The QI Macros will perform all of the calculations and create the histogram for you. Saves lots of time over out of the box Excel and manual calculations.
  2. Process Capability Metrics and Bell Curve: The QI Macros will prompt you for spec limits and will calculate Cp Cpk Pp Ppk for you. It also calculates twenty other metrics like % defects, sigma, etc. and draws a bell curve for you.
  3. Interactive Features: QI Macros histograms are easy to update and experiment with:
    • change bin intervals and number of bars
    • change spec limits and see how that impacts Cp Cpk and Pp PpkĀ 

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Create these charts and diagrams yourself in just seconds using the QI Macros for Excel...
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