Histogram Charting
What is a
Histogram
Chart?
The Histogram is a bar chart displaying the pattern of variation resulting from the operation of the process.
Mathematically Histograms are an approximate representation of the distribution of numerical data.
Interpretation: Four key aspects can easily be seen from this graphical representation of the data.
- Characteristics like, peaks and spread, symmetry and sample size.
- Multiple peaks and Outliers
- Assessing distribution fit for a theoretical distribution
- Comparing centers and spreads if your histogram has groups
Steps to Completing a Histogram
- Complete heading information.
- Label the graph portion of a frequency table form, starting on the left under the column “Class”.
- Choose small cell widths, preferably close to the resolution of the measuring instrument.
- Draw in horizontal dashed lines to represent the specification limits.
- Record the observations in the “Tally” column
- Record the number in the “Frequency” column.
- Complete the “Cumulative Frequency column.
- Complete the “Cumulative Percentage” column according to the formula: Cum.%= Cum. Freq. x 100/(N+1)
Note: N= total number of observations; N +1 is used to allow for sampling error. - Label the x-axis (horizontal scale across the bottom of the page) based on the “Class” values from the Frequency Table.
- Label the y-axis (left-hand vertical column) with frequency.
- Retrieve information from the “Frequency column” of the Frequency Table, and draw a horizontal line at the appropriate frequency level for each cell.
- Draw in vertical dashed lines to represent the specification limits.
