Histogram | Histogram Chart | What is Histogram in 7 QC Tools?

Histogram | Types of Histogram Chart | What is Histogram in 7 QC Tools?

What is Histogram in 7 QC Tools?

→ Histogram is a type of bar chart representing the frequency distribution of the data from a process.
→ Only one parameter can be used for constructing this chart.
→ The data should be variable data, i.e., measured on a continuous scale. ( eg. weight, time, temperature, dimensions, speed, etc.)
→ It is a very good tool of The Basic Seven QC Tools.

➤ Key Concept of Histogram in 7 QC Tools:

➥ The Key Concept are:

  1. Data always have a variation
  2. Variation has Pattern
  3. Patterns can be seen easily when summarized pictorially

Seven Simple Steps for construction of Histogram:

➤ Seven Steps are:

  1. Collect and record the data Points
  2. Compute the Range
  3. Determine the numbers of intervals
  4. Determine the interval width
  5. Summarize the data
  6. Construct the Graph
  7. Analyze the pattern of the chart

➥ Step 1. Collect and Record data:

→ For better analysis and forecasting of process collect at least 100 data points.
→ Here we take an example of 150 data points for making a graph.


Collect and Record data for Histogram in 7 QC Tools

➥ Step 2. Compute the Range:

 Largest Value        = 442
 Smallest Value      = 204
 Range of Values   =  442-204  = 238

➥ Step 3. Determine the No. of intervals required:

→ This step is for determining the no. of intervals required to construct Histogram.

➥ Step 4. Determine the interval width:

→ Step 4 is for determining the width of intervals to construct.

Determine the interval width Histogram in 7 QC Tools

➥ Step 5. Summarize the data:

→ In Step 5 we will summarize the data as per the interval and count the frequency of data as per range value for construction of Histogram.

➥ Step 6. Construct the Graph:

→ Now we will construct the Histogram by mentioning the Title, 
    Y-axis/Vertical axis = Interval width, X-axis/Horizontal axis = Interval width, 
    Graph - Trendline and Legend (if applicable).


Construct Histogram in 7 QC Tools

➥ Five Important Parameters in Histogram:

  1. Title
  2. Y-axis/Vertical axis = Interval width
  3. X-axis/Horizontal axis = Interval width
  4. Graph - Trendline
  5. Legend (if applicable)
→ For a better understanding of these 5 parameters study below graph:


5 parameters in Histogram

➥ Step 7. Analyze the Graph pattern:

→ Interpretation of the Graph
  • Process Centering.
  • Process Variation.
  • Histogram Shape.
  • Process Comparison with the specification  (Process Capability).

Types of Histogram based on Pattern in 7 QC Tools:

➤ Various types of Histogram patterns are:

➥ [A] Normal Distribution:

→ Bell Shaped Curve
→ A peak in the middle


➥ [B] Skewed Distribution:

→ A peak is off center either right or left
→ Analysis of very pure product is skewed i.e. product cannot be more than 100%.
→ The range is not set properly for process


Normal Distribution of Histogram in 7 QC Tools

➥ [C] Double-Peaked / Bi-modal:

→ Two Bell Shaped distributions.
→ Maybe a combination of two processes or two shift’s data.

➥ [D] Multi Peaked / Multi-modal:

→ Maybe data of several processes are plotted on the same graph.
→ Range selection is not proper.


Double-Peaked  Bi-modal Histogram in 7 QC Tools

➥ [E] Edge Peaked:

→ Borderline doubt for inspection and take it as ok.
Instrument error.


➥ [F] Truncated or Heart-cut:

→ The supplier might mix apart within spec and out of spec. limit.
→ 100% of Incoming inspection and screening of mixed parts


Truncated or Heart-cut Histogram in 7 QC Tools

Benefits of Histogram in 7 QC Tools:

→ Summarize Large dataset Graphically
→ Confirms measurements to Specification
→ Excellent problem forecasting tool in the process
→ Assist to decision making in the ongoing process