Histogram – 7 QC Tools

Histogram is a bar graph which shows the distribution of data. It shows the snap shot of the data taken from a process.


The Purpose of Histogram is to

  • Summarize large data sets graphically
  • Compare measurements to specifications
  • Assist in decision making during problem solving

The Data spread in the process is the result of the variation that exists in the process.

Histogram helps us to understand that with the prevailing variation the process is capable to produce within the specifications. This is done by just superimposing the Specification Limits with the Histogram drawn


The above example indicates that the process is well capable as the variability of the process data is well within the specification

There is always possibilities of a capable process to produce defects if the setting parameters is not in line with the specification Mean. Histogram also help to understand the setting of a process is line with the specification.


The above example indicates that the variability in the process is very less compared to the specification limits indicating that the process is a capable process. However a pile of data fall outside the specification limits producing defects. This is due to the poor setting parameters.

In statistical terms histogram Helps us to

  • know the central value of the data (setting)
  • know the spread (variation) in the data
  • assess the ability of the process to meet the specifications
  • estimate the extent of non-conformance in the process output
  • understand whether the non-conformance is due to setting or due to variation

Histogram is a powerful tool helps to understand the process in just a glimpse. It helps in decision making during  Problem solving.

Few books recommended to read on Problem solving and Histogram are given below

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