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7 QC Tools: Histogram (Video Tutorial)

What is a Histogram?

  • SPC technique
  • Graphical tool that represents the data values with the help of vertical rectangular bars
  • Height of the bars is corresponding to the frequency of the data values

Why does the Histogram use as a qc tool?

Showcases the large amount of data using vertical bars. Helps in analysing the properties of data in statistical process control such as;

  • Distribution of the data
  • Spread of the data
  • Variation in the process
  • Skewness present in the data

How do you analyse the shapes of Histogram?

  1. When the no. of vertical bars are equal in both sides of tallest bar in descending order making a normal curve like bell shape, then this implies the data has symmetrical or normal distribution
  2. When the vertical bars from the tallest bar descending and making a longer tale in right hand side, then this implies the data has right handed skewness
  3. When the vertical bars from the tallest bar descending and making a longer tale in left hand side, then this implies the data has left handed skewness
  4. When the height of the histogram is above normal then this implies the data has peaked distribution
  5. When the height of the histogram is below normal then this implies the data has flatten distribution
  6. When there are two vertical bars having equal height but taller than rest of the vertical bars then this implies the data has bimodal distribution

How do you collect the data for Histogram?

Data can be collected in two ways for Histogram;

  • 1st method is individual frequency count. In this method frequency of every observation counts individually using tally marks as shown in table 1
  • 2nd method is counting raw data into groups or in class intervals. When there are very large no. of observations, the data values are divided into class intervals of equal width and width of every class interval is the difference of upper limit and lower limit of the class interval shown in table 2

How do the ungrouped data present on graph using Histogram?

  • One hundred observations of hardness variation for a particular product shown here
  • Table 3 shows the frequency of every individual value of hardness, counting with help of tally marks
  • Now to draw the histogram, take the individual values on X– Axis, and the frequencies on Y– Axis 
  • Now draw the vertical bars corresponding to the height of the frequency of each individual hardness value
  • The no. of vertical bars showcase the spread of the data,
  • The variation in the heights of bars shows the process variation, and the shape of histogram elaborates the distribution of the data. Here it shows, the data has symmetrical distribution

How do you present the data in groups or class intervals using Histogram?

  • Table 4 shows the distribution of harness values into groups.
  • Frequency of every class interval can be counted with help of tally marks by counting the hardness values falling in that class interval
  • Now to draw the histogram, take the class intervals on X– Axis, and the frequencies on Y– Axis 
  • Now draw the vertical bars corresponding to the height of the frequency of each class interval same as in case of histogram for individual frequency data
  • Here also the shape of histogram shows, the data has symmetrical distribution

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Last updated on: 04 July, 2017

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