Home » Video Tutorials » 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

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

- 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
- 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
- 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
- When the height of the histogram is above normal then this
implies the data has peaked distribution
- When the height of the histogram is below normal then this
implies the data has flatten distribution
- 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

Data can be collected in two ways for Histogram;

- 1
^{st}method is individual frequency count. In this method frequency of every observation counts individually using tally marks as shown in table 1 - 2
^{nd}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

- 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

- 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