Running and Changing Histograms
Histograms are simply bar charts that show the distribution of your data using the number of times your data points fall into each of the bars on the histogram. When you add the upper and lower specification limits, it’s easy to see how your data fits your customer's requirements and what improvements might be necessary.
QI Macros histograms calculate key indicators of process capability including Cp, Cpk, Pp and Ppk. See our formulas page for more information about these indexes.
Running a Histogram in QI Macros SPC Software
If you have QI Macros, you can access the test data used in this example on your computer at c:/qimacros/testdata/histogram.xlsx.
To create a histogram in QI Macros follow these steps:
- The number of decimals in your raw data will drive the number of decimals shown in the results. Make sure your data is formatted to the precision you want. From Format- Cells, select Number and specify the number of decimal places you want.
- Highlight the data to be graphed (minimum of 20 data points recommended). Click on the top data cell and drag the mouse down to include just the data cells. Then, click on QI Macros Menu bar and select either the histogram or frequency histogram macro. Tip: Do NOT sort your data before running a histogram or the calculations will be incorrect:
- The histogram macro will prompt you for upper and lower spec limits. Use the default or input your spec limits. For one-sided histograms, select Cancel if there is no USL or LSL. In this example, we input 346 for the USL and 200 for the LSL:
- Then, the macro will draw the graph for you:
- To move the USL or LSL arrows or text boxes:
- Arrows: Click on each arrow and drag it to the appropriate position. To extend an arrow, click on it, then click on the handle at either end and extend the arrow.
- Text: Click on each text box and drag it to sit on top or beside its corresponding arrow.
- From the File Menu, select Save to save the graph.
- To revise the process capability calculations, switch to the Histdata data sheet. You can change the USL and/or LSL by typing over the current values in the cells provided. Excel will recalculate Cp, Cpk and Pp, Ppk and update the contents of the text boxes on the chart:
- Note that you can change the number of bars used on the chart by changing either the number of bars field OR the class width field in the Histdata sheet. Note: if you want an exact number of bars, use the class width field. Change the class width in small increments until you get the number of bars desired.
- If you change the number of bars, you may need to copy and paste the formulas down to new cells:
- Then go to the Histogram chart and see if it is still centered on the page. In this example, we changed the number of bars from 10 to 20. After the change the chart looks like this:
- Next, we need to update the Source Data the chart is graphing. Click on Chart, Source Data:
- Excel will open a window with the source data definitions in the Histdata sheet. To update the source data, click on the Series tab:
- In step 9, we copied formulas down to row 44 before we started getting bars with zero values, so we need to change all references to row 26 to 44 in the series window.
- Change all of the values for the Count series, then click on Count 2 and Distribution and change the values for these series as well.
- The updated chart should look like this:
- To add data, switch to the histdata sheet and add the new data:
You can also change the estimator used. Pooled Standard Deviation (1) is the default, but you can switch to Sbar (2) or Rbar (3) by scroling to the right and changing the value in the field shown:
QI Macros Calculations
To view formulas used in QI Macros, see our formulas page.
To view the calculations made to create the histogram, take a look at the Histdata sheet created when a macro is run:
As previously mentioned, you can add new data, change spec limits and change the number of bars in this sheet.